Glossary

This glossary covers a wide range of terms used for discussing writing systems.

Note: Should we reference things like Shoebox, Lingualinks, WorldPad, PERL, SGML, SDF? or is it time to remove them? Some of them are cross referenced like PERL and “Practical Extraction and Reporting Language” so be sure to remove both if we remove them.

Note: Decide whether to add these: GlyphsApp, smith, Docker, Container, Image, anvil, WSL, TeamCity, Google Fonts,

Note: Added some words that need definitions added.

A	B	C	D	E	F	G	H	I	J	K	L	M
N	O	P	Q	R	S	T	U	V	W	X	Y	Z

A

Term	Definition
AAT	see Apple Advanced Typography.
abjad	a form of writing in which the vowels are omitted or optional, such as Hebrew and Arabic scripts.
abstract character	a unit of information used for the organization, control or representation of textual data. Abstract characters may be non-graphic characters used in textual information systems to control the organization of textual data (e.g. U+FFF9 INTERLINEAR ANNOTATION ANCHOR), or to control the presentation of textual data (e.g. U+200D ZERO WIDTH JOINER).
abstract character repertoire	a collection of abstract characters compiled for the purposes of encoding. See also charset.
abugida	a form of writing in which the consonants and vowels in a syllable are treated as a cluster or unit; typical of scripts from South Asia.
advance height	the amount by which the current display position is adjusted vertically after rendering a given glyph. This number is generally only meaningful for vertical writing systems, and is usually zero within fonts used for horizontal writing systems.
advance width	the amount by which the current display position is adjusted horizontally after rendering a given glyph.
affrication	the phonological process by which a simple stop, such as [t], is converted to an affricate, such as [tʃ]. For example, in some dialects of British English the word “tuna” is pronounced [tʃu:na], the first consonant having been affricated.
allograph	a variant form of a grapheme. For example, in Arabic, each letter has an initial, medial and final form depending on its position within a word. These are considered allographs of a single grapheme. The term may also be used in cases where a group of letters is used as an alternative for a single grapheme, as in the English word cough, where the sequence g+h represents the same sound as the single grapheme f.
allophone	a variant of a phoneme. It is not distinctive, that is, substituting one allophone for another of the same phoneme will not change the meaning of the word, although it will sound unnatural. Broadly speaking, the test to determine whether two sounds are allophones of the same phoneme, or separate phonemes, is to see whether they are in complementary distribution, that is, when two phonological elements are found only in two complementary environments. For example, in English /ph/ only occurs syllable-initially when followed by a stressed vowel, but /p/ occurs in all other environments. This is illustrated by the words pin /phin/ and spin /spin/. Therefore, /ph/ and /p/ are seen to be in complementary distribution, and therefore allophones of the phoneme [p]. This test is not foolproof; some sounds are in complementary distribution but are not considered to be allophones. For example, in English /h/ only occurs syllable-initially and /ʔ/ only occurs syllable-finally. However they are phonetically so different that they are still considered to be separate phonemes. One allophone can be assigned to more than one phoneme, as illustrated in some North American English dialects, where the phonemes /t/ and /d/ can both be changed into the allophone [ɾ].
alphabet	a segmental writing system having symbols for individual sounds, rather than for syllables or morphemes. In a true alphabet, consonants and vowels are written as independent letters, in contrast to an abugida or an abjad. In a perfectly phonemic alphabet, phonemes and letters would be predictable in both directions; that is, the sound of a word could be predicted from its spelling and vice-versa. A phonetic alphabet is also predictable in this way, however it uses separate letters for separate allophones, whereas a phonemic alphabet may describe allophones of the same phoneme using a single letter.
anchor point	see attachment point.
Apple Advanced Typography	a smart font rendering technology developed by Apple.
articulator	any vocal organ used to form a speech sound. For example, the articulators used to form the sound [f] are the lower lip (a labial articulator) and the upper teeth (a dental articulator), so is described as a ‘labiodental’ sound. Active articulators are those organs which can move - for example the tongue - and passive articulators are those which are fixed - for example the roof of the mouth.
ascent	the distance between the top of the line of text and the baseline, or the distance from the baseline to the top of the highest glyph in a font.
ASCII	a standard that defines the 7-bit numbers (codepoints) needed for most of the U.S. English writing system. The initials stand for American Standard Code for Information Interchange. Also specified as ISO 646-IRV.
attachment point	a point defined relative to a glyph outline such that if two attachment points on two glyphs are positioned on top of each other, the glyphs are positioned correctly relative to each other. For example, a base character may have an attachment point used to position a diacritic, which would also have a corresponding attachment point. Also called anchor point.

B

Term	Definition
baseline	the vertical point of origin for all the glyphs rendered on a single line. Roman scripts have a baseline on which the glyphs appear to ‘sit’, with occasional descenders below. Many Indic scripts have a hanging baseline, in which the bulk of the letters are placed below the baseline, with occasional ascenders above the line. Some scripts, such as Chinese, use a centered baseline, where the glyphs are all positioned with their centers on the baseline.
Basic Multilingual Plane (BMP)	the portion of Unicode’s codespace in which all of the most commonly used characters are encoded, corresponding to codepoints U+0000 to U+FFFF, abbreviated as BMP. Also known as Plane 0. See also Supplementary Planes and Unicode BMP.
bicameral	describes a script with two sets of symbols that correspond to each letter, most often upper- and lower-case. See also unicameral. Examples of bicameral scripts include Roman (or Latin), Greek, and Cyrillic.
bidirectionality	the characteristic of some writing systems to contain ranges of text that are written left-to-right as well as ranges that are written right-to-left. Specifically, in Arabic and Hebrew scripts, most text is written right-to-left, but numbers are written left-to-right. This can also be used to refer to text containing runs in multiple writing systems, some RTL and some LTR.
BMP	see Basic Multilingual Plane.
BOM	see byte order mark.
bounding box	the rectangular area containing the entire visual portion of a glyph but excluding the side-bearings and advance width (or height).
boustrophedon	a way of writing in which successive lines of text alternate between left-to-right and right-to-left directionality. (See also blog entry on boustrophedon)
breve	a diacritic mark (˘) shaped like a concave-up half-circle and positioned centrally above a letter. In phonetic transcription it indicates a short vowel. It is also sometimes used in other writing systems to represent language-specific sounds (usually vowels) which are not represented by other, single letters in the script. For example, Romanian uses the character Ă/ă to represent the sound [ə]. Contrast with macron and caron.
byte order mark (BOM)	the Unicode character U+FEFF ZERO WIDTH NO-BREAK SPACE when used as the first character in a UTF-16 or UTF-32 plain text file to indicate the byte serialization order, i.e. whether the least significant byte comes first (little-endian) or the most significant byte comes first (big-endian). Byte order is not an issue for UTF-8, though the byte order mark is sometimes added to the beginning of UTF-8 encoded files as an encoding signature that applications can look for to detect that the file is encoded in UTF-8. See also Unicode BOM.

C

Term	Definition
candrabindu	a diacritic used in a number of Indic scripts, usually to represent nasalization. It is generally written with a dot inside the lower half of a circle.
caron	a diacritic mark (ᵛ), also called an inverted circumflex or haček, shaped like a pointed breve or superscripted ‘v’ and positioned centrally above a letter. For some Central European languages, the caron combined with certain letters (lower-case ť, ď, ľ, and upper-case Ľ) is reduced to a small stroke resembling but not identical to an apostrophe. It is used in a number of Baltic, Slavic, and Finno-Lappic languages to indicate that a sound has been palatalized, iotatized (mixed with the approximant /j/), or articulated with the tongue near or touching the back of the alveolar ridge. It is also used in some African writing systems and in Chinese pinyin romanization to mark tone.
Cascading Style Sheets (CSS)	one of two stylesheet languages used in Web-based protocols (the other is XSL). CSS is mainly used for rendering HTML, but can also be used for rendering XML. It is much less complex than XSL, i.e., it can only be used when the structure of the source document is already very close to what is desired in the final form.
character	(1) a symbol used in writing, distinguished from others by its meaning, not its specific shape; similar to grapheme. It relates to the domain of orthographies and writing. See orthographic character. (2) specific to the implementation of computers and other information systems. See also abstract character and encoded character.
character encoding form	a system for representing the codepoints associated with a particular coded character set in terms of code values of a particular datatype or size. For many situations, this is a trivial mapping: codepoints are represented by bytes with the same integer value as the codepoint. Some encoding forms may represent codepoints in terms of 16- or 32-bit values, though, and some 8-bit encoding forms may be able to represent a codespace that has more than 256 codepoints by using multiple-byte sequences. Most encoding forms are designed specifically for use in connection with a particular coded character set; e.g. UTF-8 is used specifically for encoded representation of the Universal Character Set defined by Unicode and ISO/IEC 10646. Some encoding forms may be designed for use with multiple repertoires, however. For example, the ISO 2022 encoding form supports an open collection of coded character sets and specifies changes between character sets in a data stream using escape sequences. See also Unicode TR17 Character Encoding Form.
character encoding scheme	a character encoding form with a specific byte order serialization (relevant mainly for 16- or 32-bit encoding forms)
character set encoding	a system for encoded representation of textual data that specifies the following: (1) a coded character set, (2) one or more character encoding forms and (3) one or more character encoding schemes.
character variant	in OpenType, a typographic alternative for one character or a set of closely related characters. See also stylistic set.
charset	an identifier used to specify a set of characters. Used particularly in Microsoft Windows and TrueType fonts, and in HTML and other Internet or Web protocols to refer to identifiers for particular subsets of the Universal Character Set.
CJKV (Chinese, Japanese, Korean and Vietnamese)	the significance of this grouping of languages is that all have writing systems that use Han ideographic characters.
CLDR	The Common Locale Data Repository. An extensive repository of locale data, where a locale is a language, spoken in a particular country, written in a particular script. The CLDR is designed to provide key building blocks for software to support the world’s languages, and is hosted by the Unicode Consortium.
cmap	character-glyph map: the table within a font containing a mapping of codepoints (characters) to glyph ID numbers. In an Unicode-based font the codepoints are Unicode values; in other fonts they correspond to other encodings.
coda	The part of the syllable that follows the nucleus. e.g. the [s] in [beɪs]
coded character set	an abstract character repertoire together with an assignment of numeric codepoints for each character; a collection of encoded characters. Also called a codepage.
codepage	(1) synonym for coded character set. (2) synonym for character set encoding; i.e. In some contexts, codepage is used to refer to a specification of a character repertoire and an encoding form for representing that repertoire. (3) In some systems, a mapping between encoded characters in Unicode and a non-Unicode encoding form; e.g. Microsoft Windows codepage 1252.
codepoint	a numeric value used as an encoded representation of some abstract character within a computer or information system. Codepoints are integer values used to represent particular characters within a particular encoding.
codespace	the full range of numeric codepoint values allowed in a coded character set.
colometry	in writing, the distribution of text into sense lines, so that a new clause starts on new line.
complementary distribution	the relation between two or more variants of a given entity (sound, morpheme etc) which appear in mutually exclusive contexts. x and y are said to be in complementary distribution if x is used where y is not used, y is used where x is not used, and between the two environments where x and y are used, every potential environment for that element is covered. In English, the plural suffix is pronounced [s] after non-sibilant voiceless consonants as in trucks [tr∧ks], [z] after voiced non-sibilants and vowels, as in trees [tɹi:z], and [əz] after sibilant consonants as in buses [b∧səz]. Therefore the three variants of the plural suffix are said to be in complementary distribution. Sounds which are in complementary distribution are known as allophones; morphemes which are in complementary distribution are known as allomorphs. The counterparts to complementary distribution are contrastive distribution and free variation. Complementary Distribution.
complex script	a script characterized by one or more of the following: a very large set of characters, right-to-left or vertical rendering, bidirectionality, contextual glyph selection (shaping), use of ligatures, complex glyph positioning, glyph reordering, and splitting characters into multiple glyphs.
conjunct	a ligature, in particular, a ligature representing a consonant cluster in an Indic script.
continuant	in phonetics, a speech sound which is produced without complete closure of the vocal tract. That is, any sound other than a stop, or any sound which can be articulated continuously.
contrastive distribution	the relation between two or more variants of a given entity (sound, morpheme etc) which distinguish between units. For example, a pair of phonemes such as [p] and [b], which distinguish between the words pin [pɪn] and bin [bɪn] are said to be in contrastive distribution. Suffixes such as -ed and -s which distinguish between the past tense (e.g. walked) and the present tense (e.g. walks) are similarly said to be in contrastive distribution. The counterparts to contrastive distribution are complementary distribution and free variation. See also Contrastive Distribution.
CoreText	a shaping engine for laying out text and handling fonts on iOS and macOS.
creole	a fully-functioning language which has developed as a result of interaction between two (or more) parent languages. Often, a creole develops from a pidgin if the pidgin is used for long enough for a sophisticated grammar and vocabulary to evolve, and if the pidgin acquires native speakers (if children learn it as their first language).
CSS	see cascading style sheets.

D

Term	Definition
DBL	see Digital Bible Library.
Digital Bible Library	an archive and a licensing management environment, maintained by SIL Global, which gathers, validates, and safeguards digital scripture translation and publication assets.
DDL	see digitally disadvantaged language.
digitally disadvantaged language	a language which is not well supported on the most popular devices, operating systems, browsers and mobile applications.
dead key	a key in a particular keyboard layout that does not generate a character, but rather changes the character generated by a following keystroke. Dead keys are commonly used to enter accented forms of letters in writing systems based on Roman script.
deep encoding	see semantic encoding.
defective	with regard to writing systems, a writing system which does not represent all the distinctive sounds of the language it represents.
deprecate/deprecated	no longer encourage(d) for use. There is generally a better solution than the one that is deprecated. This term is often applied to codepoints, language tags, programming commands, etc.
descent	the distance between the bottom of the line of text and the baseline, or the distance from the baseline to the bottom of the lowest glyph in a font.
determinative	in semantics, a class of words that indicates, specifies or limits a noun, such as the definite or indefinite article, the genitive (possessive) marker, or cardinal numbers. In logographic writing systems, determinatives are one of three types of logograph, the other two being phonographs and ideographs. Determinatives generally have no spoken equivalent but perform a grammatical function to disambiguate between multiple possible interpretations of a phonograph or ideograph.
diacritic	a written symbol which is structurally dependent upon another symbol; that is, a symbol that does not occur independently, but always occurs with and is visually positioned in relation to another character, usually above or below. Diacritics are also sometimes referred to as accents. For example, acute, grave, circumflex, etc.
diaeresis	a diacritic mark (¨), also called tréma, commonly placed over the second of two adjacent vowels to indicate that they are to be pronounced as separate sounds rather than as a diphthong, as in the English word naïve. It can also be used to indicate that an otherwise unpronounced vowel is to be pronounced, as in the English name Brontë or the French word cigüe. In Welsh orthography, it is often written on the first of two adjacent vowels to indicate that the first vowel bears stress. The same mark when used over a single vowel in Germanic writing is called an umlaut and indicates a change in vowel quality.
digraph	a multigraph composed of two components.
diphthong	in phonetics, a complex speech sound occupying one syllable, which begins with one vowel and ends with another. For example [eɪ] in British (RP) pronunciation of the word lane. See also monophthong.
DirectWrite	a shaping engine for laying out text and handling fonts on Windows.
display encoding	See presentation-form encoding.
distinctive	also contrastive. An element which makes a distinction between units. In phonology, a process or a pair of sounds, the alternation of which changes the meaning of a word. See also phoneme, minimal pair. For example, voicing is distinctive in most non-tonal languages, as illustrated by the difference between English fan and van, or German Kern and gern.
document	a collection of information. This includes the common sense of the word, i.e. an organization of primarily textual information that can be produced by a word processing or data processing application. It goes beyond this, however, to include structured information held within an XML file. Each XML file is considered to contain one document, whatever the structure and type of that information.
Document Type Definition (DTD)	a markup declaration used by SGML and XML that contains the formal specifications, or grammar, of an SGML or XML document. One use of the DTD is to run a validation process over an XML file, which indicates if it matches the DTD, or if not, provides a listing of each line at which the file fails some part of the required structure.
DTD	see Document Type Definition.

E

Term	Definition
em square	the square grid which is the basis for the design of all glyphs within a given font; so called because it historically corresponded to the size of the letter M. When rendering, the requested point size specifies the size of the font’s em square to which all glyphs are scaled.
em units	the coordinates in which points in a glyph are defined. An important number is the number of em units in the em square.
encoded character	an abstract character in some repertoire together with a codepoint to which it is assigned within a coded character set. Encoded characters do not necessarily correspond to graphemes.
encoding	(1) synonym for a character encoding form. (2) synonym for a character set encoding. This usage is common, especially in cases in which distinctions between a coded character set and a character encoding form is not important (i.e. 8-bit, single-byte implementations). Someone might think of an encoding as simply a mapping between byte sequences and the abstract characters they represent, though this model is not adequate to describe some implementations, particularly CJKV standards, or Unicode and ISO/IEC 10646.
Extensible Markup Language (XML)	a standard for marking up data so as to clearly indicate its structure, generally in a way that indicates the meaning of different parts of it rather than how they will be displayed. See https://www.w3.org/XML/ for details.
Extensible Stylesheet Language (XSL)	a language for expressing stylesheets. It consists of two parts: XSL transformations (XSLT) and an XML vocabulary for specifying formatting semantics. See https://www.w3.org/Style/XSL/ for full details.
Extensible Stylesheet Language Transformations (XSLT)	a language used to convert one XML document into another. See https://www.w3.org/TR/xslt/ for full specifications.

F

Term	Definition
featural writing system	a writing system in which phonetic features, rather than phones (sounds), are represented. For example, there might be a symbol to represent the feature bilabial (a sound produced with both lips), a symbol to represent the feature voiced, and a symbol to represent the feature stop. These symbols would be combined to represent the sound [b]. The closest functioning writing system to this is the Korean Hangul, in which many of the strokes making up the symbols represent place or manner of articulation. Some writing systems used for representing signed languages also contain symbols which stand for particular features of signs. In this case, the symbol often visually resembles the feature it represents, such as direction of movement.
feature	a way of indicating variant renderings for a particular string using the same font; for example, enabling some ligature replacements or not.
font	a file containing a collection of glyphs and related supporting information used to render text.
free variation	the relation between entities which have similar distributions but which are not distinctive, that is, they are interchangeable without changing the meaning of the word or sounding unnatural. It can apply to sounds, for example, in many southern dialects of British English, inter-vocalic [ʔ] and [t] are in free variation in words such as butter [b∧ʔə] / [b∧tə]. It can also apply to entire words, for example the concept dreamPAST in English can be pronounced either [dri:md] or [drɛmpt] with no change in meaning. Free variation can also apply to writing systems, with regard to variants of symbols. Mongolian uses largely predictable variant forms of many letters; these are encoded in Unicode in such a way that the rendering system can select the correct form. However, some letter forms are unpredictable and in free variation; in this case a free variation selector must be appended manually by the user to indicate to the rendering system which form is required.

G

Term	Definition
GDL	See Graphite.
gemination	in phonetics, consonant lengthening, usually by about a time-and-a-half of the length of a ‘short’ consonant. Geminated fricatives, trills, nasals and approximants are simply prolonged. In geminated stop stops, the ‘hold’ is prolonged. In some languages, such as Japanese, Hungarian, Arabic, Italian and Finnish, gemination is distinctive, but in most it is not. In languages where it is distinctive, it is usually restricted to certain consonants. English contains very few words in which gemination affects the meaning; among these are unnamed vs. unaimed or, in some dialects _sixths- /siks:/ vs. six /siks/ (source: John Lawler, University of Michigan). In some languages, consonant length and vowel length depend on each other. For example in Swedish and Italian a short vowel must be followed by a long consonant (geminate), whereas a long vowel must be followed by a short consonant.
glyph	a shape that is the visual representation of a character. It is a graphic object stored within a font. Glyphs are objects that are recognizably related to particular characters and which are dependent on particular design (i.e. g, g and g are all distinct glyphs). Glyphs may or may not correspond to characters in a one-to-one manner. For example, a single character may correspond to multiple glyphs that have complementary distributions based upon context (e.g. final and non-final sigma in Greek), or several characters may correspond to a single glyph known as a ligature (e.g. conjuncts in Devanagari script). (For more information on glyphs and their relationship to characters, see ISO/IEC TR 15285.)
glyph ID	the unique number within a font identifying a single glyph.
glyph outline	a series of curves describing the shape of a glyph. The renderer will fill in this outline to make a solid glyph appear.
grapheme	a character or sequence of characters that functions as a distinct unit within an orthography. A grapheme may be a single character, a multigraph, or a diacritic, but in all cases graphemes are defined in relation to the particular orthography. Most graphemes represent a single phoneme, but some represent a sequence of phonemes. For instance, the character sequence ‹ch› is often used to represent the phoneme /tʃ/ in English, while the single letter ‹x› usually represents the phoneme sequence /ks/. In a highly phonemic writing system, there is a close correspondence between graphemes and phonemes. English (written with Latin script) is an example of a writing system that is not highly phonemic, and therefore the mappings between graphemes and phonemes are more complex. Graphemes are often written enclosed in angle brackets (‹›).
Graphite	a package developed by SIL to provide ‘smart rendering’ for complex writing systems in an extensible way. It is programmable using a language called Graphite Description Language (GDL). Because it is extensible, it can be used to provide rendering for minority languages not supported by Uniscribe. See Graphite.

H

Term	Definition
haček	See caron.
“Hanging” verse numbers	“Hanging” numbers are so named because the verse or chapter begins in the middle of a line and rather than the number appearing at the actual beginning point they are “hung” out in the margin so as not to distract the reader.
HarfBuzz	a cross-platform shaping engine which primarily supports OpenType, but also Graphite and Apple Advanced Typography.
heterogram	a term used mostly in the study of ancient texts, referring to a special kind of a logogram consisting of the written representation of a word in a foreign language.

heteronym|homographs which, although spelled the same way, are pronounced differently and have different meanings. For example, in English ‘wind’ (noun, as in weather) and ‘wind’ (verb, to coil something). homograph|one of multiple words having the same spelling but different meanings. They may be pronounced differently (for example in English ‘tear: rip’ and ‘tear: secreted when crying’), in which case they are also heteronyms, or they may be pronounced the same (for example in American English ‘tire: cause to be fatigued’ and ‘tire: wheel of a car’), in which case they are also homophones. homophone|one of multiple words having the same pronunciation but different meanings. They may be spelled differently (for example in English ‘write’ and ‘right’), in which case they are called heterographs, or the same (for example in English ‘bark: on a tree’ and ‘bark: of a dog’), in which case they are also homographs.

I

Term	Definition
ideograph	see logograph.
IME	see input method editor.
input method	any mechanism used to enter textual data, such as keyboards, speech recognition or handwriting recognition. The most common form of input method is the keyboard. The term “input method” is intended to include all forms of keyboard handling, including but not limited to input methods that are available for Chinese and other very-large-character-set languages and that are commonly known as input method editors (IMEs). An IME is taken to be a specific type of the more general class of input methods.
input method editor (IME)	a special form of keyboard input method that makes use of additional windows for character editing or selection in order to facilitate keyboard entry of writing systems with very large character sets.
internationalization	a process for producing software that can easily be adapted for use in (almost) any cultural environment; i.e. a methodology for producing software that can be script-enabled and is localizable. Sometimes abbreviated as ‘I18N’.

J

None.

K

Term	Definition
kern	to adjust the display position whilst rendering in order to visually improve the spacing between two glyphs. For instance, kerning might be used on the word WAVE to reduce the illusion of white space between the diagonal strokes of the W, A, and V.
Keyman	an input method program which changes and rearranges incoming characters to allow easy ways of typing data in writing systems that would otherwise be difficult or inconvenient to type. See Keyman.

L

Term	Definition
labialization	the use of the lips as a secondary articulator while the rest of the oral cavity produces a different sound. Generally - but not always - this means that a sound is modified by simultaneous rounding of the lips. The term is normally used to refer to consonants; the process can also be applied to vowels, but these are more commonly referred to as rounded rather than labialized. The process is most commonly applied to velar consonants (that is, those produced with the back of the tongue against the soft palate, such as [k] or [x]). For example, in the English word cool [ku:l], the [k] is often pronounced with the lips rounded, whereas in keep< [ki:p] it is not. Labialization is extremely widespread across the world’s languages and, in some, it is distinctive.
LANGID	in the Microsoft Win32 API, a 16-bit integer used to identify a language or locale. A LANGID is composed of a 10-bit primary language identifier together with a 6-bit sub-language identifier (the latter being used to indicate regional distinctions for locales that use the same language).
language ID	a constant value within some system used for metadata identification of the language in which information is expressed. May be numeric or character based, depending on the system.
Latin script	see Roman script.
LFF	see Language Font Finder.
Language Font Finder	a micro service responder that when passed language tag will return information on a possible font to use for the orthography for that language tag. See langfontfinder.
left side-bearing	the white space at the left edge of a glyph’s visual representation, or more specifically, the distance between the current horizontal display position and the left edge of the glyph’s bounding box. A positive left side-bearing indicates white space between the glyph and the previous one; a negative left side-bearing indicates overlap or overhang between them.
ligature	a single shape or glyph that represents two or more underlying characters. See also conjunct.
locale	a collection of parameters that affect how information is expressed or presented within a particular group of users, generally distinguished from one another on the basis of language or location (usually country). Locale settings affect things such as number formats, calendrical systems and date and time formats, as well as language and writing system.
localizability	the extent to which the design and implementation of a software product allows potential for localization of the software.
localization	the process of adapting software for use by users of different languages or in different geographic regions. For purposes of this document, localization has to do with the language and script of users, and is distinct from script enabling, which has to do with the script in which language data is written. The localization process may include such modifications as translating user-interface text, translating help files and documentation, changing icons, modifying the visual design of dialog boxes, etc. Sometimes abbreviated ‘L10N’.
logical order	a way of storing characters. Roughly, the order in which characters are read or pronounced, as opposed to the order in which they appear on the page (visual order). In many cases, these are the same. However, in some left-to-right Indic scripts, certain vowels are written to the left of a consonant but pronounced after it. In these cases, the vowel character is stored as the second character in the sequence, although visually it is the first. Similarly, in bidirectional text, some portions are read from left to right and some are read from right to left, so there is a discrepancy between the order in which they appear on the page and the order in which they are read. One of the characteristics of Unicode is that characters are stored in logical order.
logograph	also called a logogram or ideograph. A written symbol representing a whole word. Technically, this is distinct from an ideogram, which represents a concept independently of words, although the two are often used interchangeably.
logographic writing system	also known as an ideographic writing system. A writing system in which each symbol represents a complete word or morpheme. The symbols do not indicate the word’s pronunciation, only its meaning. Historically, Sumerian cuneform and Egyptian hieroglyphics were logographic, but today Chinese is the only known writing system in the world that remains logographic. See also logosyllabary.
logosyllabary	a writing system in which each sign is used primarily to represent words or morphemes, with some subsidiary usage to represent syllables. Most natural logosyllabaries employ the rebus principle to extend the character set so that syllables as well as morphemes can be represented. Logosyllabaries may also include determinatives to mark semantic categories which would otherwise be ambiguous. The extent to which syllabic sounds are represented varies from one writing system to another. In instances where a relatively large number of symbols represent syllabic sounds, a logosyllabary may evolve into an abugida or an abjad as the syllabic use overtakes the logographic use.

M

Term	Definition
macron	a diacritic mark (ˉ) shaped like a short horizontal line and placed centrally above or below a symbol. Above a vowel it normally indicates that the vowel is long. Some writing systems use macron above a vowel to represent tone. Transliterations of Biblical Hebrew use macron below certain consonants to represent sounds which cannot be written as a single Latin letter. See Macron.
matres lectionis	(lit. “mothers of reading”). The use of certain consonant letters, primarily in semitic abjads to indicate vowel sounds.
metathesis	a phonological change in which the order of segments, particularly successive sounds, in a word is reversed. For example, the English word ‘ask’ was pronounced [æks] between the 5th and 12th centuries, and some dialects have reverted back to this pronunciation in modern times.
minimal pair	a pair of words distinguished by only one phoneme, for example in German /kern/ ‘centre’ and /gern/ ‘like, with pleasure’.
mnemonic keyboard	a keyboard layout based on the characters appearing on the keytops of the keyboard. See also positional keyboard.
monophthong	a vowel sound which does not change in quality as it is articulated. (Contrast with diphthong.) It can be short, as in English bed [b?d], or long, as in English bead [bi:d]. A single short monophthong is the shortest syllable in any language. The process by which monophthongs change to diphthongs or vice versa is an important factor in language change. Diphthongization in the 15th or 16th century changed the long German monophthong [i?] to [a?], as in Eis ‘ice’, and long [u?] to [a?] as in Haus ‘house’. A characteristic of Southern American English is the monophthongization of certain dipthongs such as [a?] to long [a:] in words such as kite.
mora	a unit of rhythmic measurement based syllable weight, which is distinctive in some languages. Japanese is one of the most well-documented of these languages. Short (or light) syllables are monomoraic, consisting of one mora. Long (or heavy) syllables are bimoraic, consisting of two morae. Some languages contain superheavy syllables, for example Hindi, in which a long vowel can be followed by a geminate consonant. These syllables are said to be trimoraic. The first consonant of a syllable does not represent any morae, as it does not constitute a syllable in itself. Syllable-final consonants can either form the final part of a bi- or trimoraic syllable, as is the case in Goidelic Irish, or they can represent a mora in themselves, as is the case in Japanese. Although there is a relation between syllables and morae, they are not necessarily interchangeable. For example, the Japanese word for ‘photograph’, [sjasin], consists of 2 syllables: sja + sin, but 3 morae: sja + si + n. (source: Jouji Miwa at Mora and Syllable (No longer there))
multi-language enabling	see script enabling.
multi-script enabling	see script enabling.
multi-script encoding	an encoding implementation for some particular language that is designed to enable input to and rendering from that encoding using more than one writing system. When such an implementation is used, the different writing systems are normally based on different scripts.
multigraph	a combination of two or more written symbols or orthographic characters (e.g. letters) that are used together within an orthography to represent a single sound. (Combinations consisting of two characters are also known as digraphs.)

N

Term	Definition
non-Roman script	a script using a set of characters other than those used by the ancient Romans. Non-Roman scripts include relatively simple ones such as Cyrillic, Georgian, and Vai, and complex scripts such as Arabic, Tamil, and Khmer.
normalization	transformation of data to a normal form. For historical reasons, the Unicode standard allows some characters to have more than one encoded representation. For example, á may be represented as a single codepoint, U+00E1 LATIN SMALL LETTER A WITH ACUTE, or two codepoints, U+0061 LATIN SMALL LETTER A and U+0301 COMBINING ACUTE ACCENT. A normalization scheme is used to standardize the codepoints so that every character is always represented by the same sequence of codepoints. Normalization is described in the Unicode Standard Section 5.7, Normalization.
nucleus	the central element in a syllable, usually a vowel or diphthong e.g. the [eɪ] in [beɪs]

O

Term	Definition
onset	the part of a syllable that comes before the nucleus e.g. the [b] in [beɪs]
OpenType	a smart font rendering technology developed by Microsoft and Adobe; an extension to the TrueType font specification. See also Uniscribe.
orthographic character	a written symbol that is conventionally perceived as a distinct unit of writing in some writing system or orthography.

P

Term	Definition
PDF	see Portable Document Format.
PERL	see Practical Extraction and Reporting Language.
phone	a speech sound which is identified as the audible realization of a phoneme.
phoneme	the smallest distinctive segment of sound in any language. It is actually comprised of a group of similar sounds, called allophones, which native speakers of a language may perceive as being all the same. If a pair of words exist which differ only in one phonological element (known as a minimal pair), the element in which they differ is distinctive, and represents two phonemes in the language. For example, in English, bit and pit are a minimal pair; [b] and [p] are distinct phonemes, written as /b/ and /p/. (Phonemes are conventionally written surrounded by slashes.) Phonemes are not consistent across languages; two sounds may be separate phonemes in one language and allophones in another.
phonemic inventory	an inventory of all the distinctive sounds (phonemes) in a given language, also called a phoneme inventory. A language’s phonemic inventory is not fixed over time; as the language changes, sounds which were previously allophones may become phonemes. The smallest documented phoneme inventory belongs to the Rotokas language, which uses only 11 phonemes. The largest belongs to !Xóõ, with an estimated 112 phonemes. The number of phonemes used in speech does not necessarily correspond to the number of symbols used in writing for a given language. For example, the English alphabet contains 26 letters, but the phonemic inventory numbers between 35 and 47 depending on the dialect used (source: Wikipedia). In a true phonemic script the symbols should map on a one-to-one basis to the sounds in the phonemic inventory.
phonemic script	a writing system in which each symbol tends to correspond to one phoneme. For example, the N’Ko alphabet assigns one symbol to each phoneme. Also sometimes called a phonetic script although technically this is not accurate, as a true phonetic script should represent every allophone in a language.
phonetization	see the rebus principle.
pidgin	a simplified contact language that develops and used by people who do not speak a common language but share a geographic area. Pidgins are characterized by a simplistic grammatical structure, and a limited (usually context-specific) vocabulary. They are not spoken as a first language. Given time, a pidgin may evolve into a creole.
pitch accent	one of two types of phonological accent which is realized by differences in the frequency of accented and unaccented syllables. Its counterpart is a stress accent. Phonetically, a higher pitch is due to a more rapid vibration of the vocal cords. The placement of pitch may determine the meaning of a word, for example in the case of the two Japanese words pronounced /haꜜsi/ chopsticks and /hasiꜜ/ bridge. Technically, a pitch accent language is differentiated from a true tone language in that every syllable is characterized by a particular tone in the latter, but only some in the former. However, this distinction is often lost in practice. Some languages, such as Welsh, have a pitch accent which does not differentiate the meaning of words. Accents may or may not be marked in writing, depending on the orthographic conventions of a particular language.
plain text	textual data that contains no document-structure or format markup, or any tagging devices that are controlled by a higher-level protocol. The meaning of plain text data is determined solely by the character encoding convention used for the data.
plane	in Unicode, a range of 64K codepoints. Plane zero is the original 64K codepoints that can be represented in a single 16-bit character. See also Basic Multilingual Plane, supplementary planes, and surrogate pair.
Portable Document Format (PDF)	a particular file format for the storage of electronic documents in a paged form. Created by Adobe around their Adobe Acrobat product. Usually created from a Postscript page description.
positional keyboard	a keyboard layout defined in terms of the relative positions of keys rather than what they have printed on them. See also mnemonic keyboard.
Postscript	a page description language defined by Adobe. Originally implemented in laser printers so pages were described in terms of line drawing commands rather than as a bitmap.
Postscript font	a font in a format suitable for use within a Postscript document. There are many types. Type 1 is the most common and is what is meant most commonly when people refer to Postscript fonts. There are also ways of embedding other font formats into a Postscript document. For example a Type 42 font is a TrueType font formatted for use within a Postscript document. Type 1 fonts differ in the way their outlines are described from TrueType fonts.
Postscript name	a name associated with a glyph by the font’s designer. Originally a name assigned by Adobe to certain standard glyphs.
Practical Extraction and Reporting Language (PERL)	an interpreted programming language particularly strong for text processing.
prenasalization	the addition of a short nasal onset to another consonant sound produced at the same place. Prenasalized sounds behave phonetically as a single consonant, not as a cluster. The African Bantu languages are famous for having prenasalized consonants, for example in the Lingala words /mboka/ hill, /ndako/ house, and /ŋgomba/ hill.
presentation-form encoding	a character encoding system in which the abstract characters that are encoded match one-for-one with the glyphs required for text display. Such encodings allow correct rendering of writing systems on ‘dumb’ rendering systems by having distinct codepoints for contextual forms, positional variants, etc. and are designed on the basis of rendering needs rather than on the basis of character semantics (the linguistically relevant information). Also known as glyph encoding, display encoding or surface encoding; distinguished from semantic encoding.
Private Use Area (PUA)	a range of Unicode codepoints (E000 - F8FF and planes 15 and 16) that are reserved for private definition and use within an organization or corporation for creating proprietary, non-standard character definitions. For more information see The Unicode Consortium, 1996, pp. 619 ff.
PUA	see Private Use Area.
Python	a high-level, general-purpose programming language that often used in font and writing system resource development.

Q

None.

R

Term	Definition
rasterizing	converting a graphical image described in terms of lines and fills into a bitmap for display on an imaging device.
rebus principle	also known as phonetization. The use of a pre-existing logograph to represent a syllabic sound having the same sound as, but a different meaning from, that of the word originally represented. The rebus principle is especially useful for representing function words, proper names, and other words which would otherwise be difficult to depict. A well-known example is the Egyptian use of the symbol representing “swallow” (pronounced wr) also being used ro represent the word “big” (which was also pronounced wr). A symbol used in this way is called a rebus. The rebus strengthens the phonetic aspect of a logographic writing system by exploiting the phonetic similarities between words. If a logographic writing system is fully (or almost fully) phonetized, it may become an abugida or an abjad. Other times, it is only partially phonetized and develops into a logosyllabary.
regression test	a test (usually a whole set of tests, often automated) designed to check that a program has not ‘regressed’, that is, that previous capabilities have not been compromised by introducing new ones.
render	to display or draw text on an output device (usually the computer screen or paper). This usually consists of two processes: transforming a sequence of characters to a set of positioned glyphs and rasterizing those glyphs into a bitmap for display on the output device.
right side-bearing	the white space at the right edge of a glyph’s visual representation, or more specifically, the distance between the display position after a glyph is rendered and the right edge of the glyph’s bounding box. A positive right side-bearing indicates white space between the glyph and the following one; a negative right side-bearing indicates overlap or overhang between them.
Roman script	the script based on the alphabet developed by the ancient Romans (“A B C D E F G …”), and used by most of the languages of Europe, including English, French, German, Czech, Polish, Swedish, Estonian, etc. Also called Latin script.
rounding	see labialization.
ruby character	a small, annotative character which is written above or to the right of a Chinese character to indicate its pronunciation. Ruby characters are usually written in the Latin, Japanese, or Bopomofo script.

S

Term	Definition
schema	in markup, a set of rules for document structure and content.
script	a maximal collection of characters used for writing languages or for transcribing linguistic data that share common characteristics of appearance, share a common set of typical behaviours, have a common history of development, and that would be identified as being related by some community of users. Examples: Roman (or Latin) script, Arabic script, Cyrillic script, Thai script, Devanagari script, Chinese script, etc.
Script Description File (SDF)	a file describing certain kinds of complex script behaviour, used to control a rendering engine to which it has given its name. Created by Tim Erickson and used in Shoebox, LinguaLinks, and ScriptPad. None of these products are in use anymore.
script enabling	providing the capability in software to allow documents to include text in multiple languages or scripts, and to handle input, display, editing and other text-related operations of text data in multiple languages and scripts. Script enabling has to do with the script in which language data is written, as opposed to localization, which has to do with the language and script of the user interface.
SDF	see Script Description File.
segmental writing system	one of two categories of phonologically-based (that is, not logographic or featural) writing systems, the other being syllabic writing systems, or syllabaries. Segmental writing systems represent consonants and vowels, rather than whole syllables, as individual units. Alphabets, abugidas, and abjads are all classed as segmental writing systems. There is potential for confusion over the inclusion of abugidas in this category, as each character in this type of script does represent a full syllable. However, individual consonants and vowels are acknowledged as discrete elements in that there is a systematic graphic similarity between characters which represent syllables sharing a particular consonant or vowel. As a test to determine whether a writing system is a syllabary or a segmental abugida, if it is syllabic there will be no systematic visual similarity between, for example, the characters or character sequences representing [ka], [ke], [ko], or [ki], [pi], [ti], but in an abugida there will be.
semantic encoding	an encoding that has the property of one codepoint for every semantically distinct character (the linguistically relevant units). In general, such encodings require the use of ‘smart’ rendering systems for correct appearance to be achieved, but are more appropriate for all other operations performed on the text, especially for any form of analysis. Also known as deep encoding; distinguished from presentation-form encoding.
SFM	see Standard Format Marker.
SGML	See Standard Generalized Markup Language.
side bearing	the white space at the edge of a glyph; see left side-bearing, right side-bearing. There can also be top and bottom side bearings, of use when rendering text vertically.
SIL Locale Data repository	SIL’s repository of selected locale data. See sldr.
SLDR	see SIL Locale Data repository.
smart font	a font capable of performing transformations on complex patterns of glyphs, above and beyond the simple character-to-glyph mapping that is a basic function of font rendering (see cmap). The information specifying the smart behavior is typically in the form of extra tables embedded in the font, and will generally allow layered transformations involving one-to-many, many-to-one, and many-to-many mappings of glyphs.
shaping	the process of chosing the correct form or shape of characters in order to produce correct rendering.
shaping engine	a software component that uses knowledge about script behavior to select the correct shapes of characters. Examples include Harfbuzz, DirectWrite, CoreText, and the Universal Shaping Engine. The OpenType smart font system depends on an external shaping engine to provide knowledge of the appropriate shapes of characters, while in the case of the Graphite and AAT systems the script knowledge is stored in the font.
smart rendering	a rendering process that uses a smart font.
sort key	a sequence of numbers that when appropriately processed using a particular standard algorithm will position the corresponding string in the correct sort position in relation to other strings. The sort key need not correspond one number to one codepoint in the input string.
Standard Format Marker (SFM)	an element of a proprietary format developed by SIL International and used by some linguistic software applications. A standard format marker begins with a backslash (\); for example, \p would represent a paragraph tag. It is possible (and even probable) that SFMs in a single document have different character encodings. When converting to one encoding (Unicode) these must be converted with different mapping files.
Standard Generalized Markup Language (SGML)	a notation for generalized markup developed by the International Organization for Standardization (ISO). It separates textual information from the processing function used for formatting. It was found difficult to parse, due to the many variants possible, and so XML was developed as a subset to resolve the ambiguities and to make parsing easier.
smith	a Python-based framework for building, testing and maintaining WSI (Writing Systems Implementation) components such as fonts. See smith.
stop	also called a plosive. In phonology, a speech sound whose production involves a complete blockage of the air flow. This may include only consonants in which the air flow is blocked through both the mouth and the nose, such as [p] or [k], or those in which the air flow is blocked through the mouth only, such as [m] or [n]. Sounds in which the airflow is blocked through both the mouth and the nose cannot be articulated continuously.
stress accent	one of two types of phonological accent by which one syllable is heard to be more prominent than others, its counterpart being a pitch accent. Phonetically, stress is due to a difference in length, volume, vowel quality, or a combination of these. These differences are thought to reflect a greater muscular energy in the production of the stressed syllable. The placement of stress may determine the meaning of a word, for example in the case of the two English words /conˈtent/ and /ˈcontent/. Accents may or may not be marked in writing, depending on the orthographic conventions of a particular language.
stylistic set	a collection of typographic alternatives for a set of glyphs. The variants generally apply across a braoder range of characters than might be implemented using a character variant.
supplementary planes	Unicode Planes 1 through 16, consisting of the supplementary code points, corresponding to codepoints U+10000 to U+10FFFF. In The Unicode Standard 3.1, characters were assigned in the supplementary planes for the first time, in Planes 1, 2 and 14. See also Basic Multilingual Plane.
suprasegmental	a unit or feature whose domain extends over more than one minimal element. For example, stress is classed as a suprasegmental feature because its domain is a whole syllable, comprised of the smaller minimal elements consonants and vowels. Suprasegmental features may be marked in writing; in these cases, the area in which they are written is called the suprasegmental box.
surface encoding	see presentation form encoding.
surrogate pair	a mechanism in the UTF-16 encoding form of Unicode in which two 16-bit code unites from the range 0xD800 to 0xDFFF are used to encode Unicode supplementary plane characters, i.e. with Unicode scalar values in the range U+10000 to U+10FFFF.
syllabary	a form of writing in which the symbols represent syllables—most commonly a vowel-and-consonant combination. A syllabary differs from an abugida in that there are no distinct elements of the symbols to correspond to the syllable’s phonemes.
syllograph	part of a syllabary, a character which represents a syllable. The term is used almost exclusively to refer to a character from the Ethiopic script, though this tendency does not preclude its use in other contexts.
symbol-encoded font	a font used either for non-orthographic collections of shapes (such as Wingdings) or for legacy orthographies (e.g., SIL Ezra, SIL Galatia, SIL IPA) created prior to availablility of Unicode-based solutions. Symbol-encoded fonts encode characters in the Private Use Area, typically U+F020 .. U+F0FF.

T

Term	Definition
tokenization	the process of analysing a string into a contiguous sequence of smaller units: for example, word breaking or syllable breaking or the creation of a sort key.
tone	a unit belonging to a set characterized primarily by differences or changes in the levels of pitch. In a tone language, tone is used to distinguish each syllable, as illustrated by the three Ngbaka words /mà/ (with a low tone) magic, /mā/ (mid tone) I, and /má/ (high tone) to me. Tone can also be used in a system of intonation; for example, in English a rising tone may indicate surprise while a falling tone may indicate disappointment.
transcription	a written representation of spoken material.
transliteration	conversion of written characters into a different script. Often this would be from a non-Roman script to Latin, especially IPA.
TrueType font	font format used primarily in Windows and on the Mac, allows for glyph scaling and hinting.
TypeTuner Web	a service which allows the user to download customized versions of SIL’s fonts. See TypeTuner Web.

U

Term	Definition
umlaut	a diacritic mark (¨) used to represent fronting or rounding of a vowel, particularly in Germanic languages. For example, in German an umlaut changes a [a] to ä [æ]. In modern computer systems, umlaut and diaeresis are represented identically with a pair of dots, although in handwritten texts umlaut can vary from two short vertical lines to a single horizontal line over the vowel.
uncial	a style of writing entirely in single stroke, rounded upper case forms, commonly found in European texts from the 4th to the 8th century. Uncial writing is the predecessor of modern capital letters.
unicameral	describes a script with only one set of symbols per letter. See also bicameral.
Unicode	an industry-wide character set encoding standard that aims eventually to provide a single standard that supports all the scripts of the world. Unicode is closely related to ISO/IEC 10646.
Unicode Scalar Value (USV)	a number written as a hexadecimal (base 16) value that serves as the codepoint for Unicode characters. Characters in the BMP are written with four hex digits, eg: U+0061, U+AA32. Characters in supplementary planes use five or six digits.
Uniscribe (Unicode Script Processor)	due to technical limitations in OpenType, it is necessary to pre-process strings before applying OpenType smart behaviour. Microsoft uses a particular DLL (Dynamic Link Library) called Uniscribe to do this pre-processing. Uniscribe does all of the script specific, font generic processing of a string (such as reordering) leaving the font specific processing (such as contextual forms) to the OpenType lookups of a font.
Universal Character Set (UCS)	the coded character set defined by Unicode and ISO/IEC 10646, intended to support all commonly used characters from all writing systems, current and past.
Universal Shaping Engine	a shaping engine for use with a complex scripts that aren’t supported by one of the dedicated engines that support a single script.
USE	see Universal Shaping Engine.
USV	see Unicode Scalar Value.
UTF-16	an encoding form for storing Unicode codepoints in 16-bit words. It includes the concept of surrogate pairs to encode values from U+10000 - U+10FFFF as two 16-bit words.
UTF-32	an encoding form for storing Unicode codepoints in 32-bit words. Since 32 bits encompasses the entire range of Unicode, every codepoint is encoded as a single 32-bit word. See Unicode Technical Report #19.
UTF-8	an encoding form for storing Unicode codepoints in terms of 8-bit bytes. Characters are encoding listing sequences of 1-4 bytes. Characters in the ASCII character set are all represented using a single byte. See https://www.unicode.org/faq/utf_bom.html.

V

Term	Definition
vigesimala	a numeral system based on the number twenty (as opposed to a decimal counting system which is based on the number ten). In the context of writing systems this usually means that twenty individual numeral symbols are used. Vigesimal systems are used in many languages in Europe, Asia, South America and Africa.
virama	the generic name for a written symbol, particularly common in Brahmic abugidas, for silencing the inherent vowel that otherwise occurs with every consonant character. The shape of the symbol varies from script to script, but it is often a diacritic, written above, below or alongside the consonant which it modifies.
virtual keyboard	a software component that allows a user to enter characters, often characters which cannot be accessed through a standard physical keyboard. Virtual keyboards can be useful for multilingual users who need to switch between different character sets.
Visual OpenType Layout Tool (VOLT)	a tool to build OpenType tables and add them to a font.
visual order	a way of storing characters so that the order in which they are stored corresponds to the order in which they appear on the page, as opposed to the order in which they are read. In many cases these are the same, but the distinction is particularly pertinent in bidirectional or mixed text, as the order in which the characters appear on the page does not necessarily correspond to the order in which they are pronounced. Older legacy font encodings tended to store characters in visual order, but Unicode-encoded fonts use logical order.
VOLT	See Visual OpenType Layout Tool.

W

Term	Definition
Web Open Font Format	a font file format for use in web pages.
WOFF	see Web Open Font Format.
WOFF2	see WOFF File Format 2.0.
WOFF File Format 2.0	a font file format that is a more compressed version of the Web Open Font Format (WOFF).
writing system	an implementation of one or more scripts to form a complete system for writing a particular language. Most writing systems are based primarily upon a single script; writing systems for Japanese and Korean are notable exceptions. Many languages have multiple writing systems, however, each based on different scripts; e.g. the Mongolian language can be written using Mongolian or Cyrillic scripts. A writing system uses some subset of the characters of the script or scripts on which it is based with most or all of the behaviours typical to that script and possibly certain behaviours that are peculiar to that particular writing system.

X

Term	Definition
x-height	the distance from the baseline of a line of text to the top of the main body of lower-case letters, that is, without ascenders or descenders. It is the height of a lower-case x, as well as a lower-case u, v, w, and z. Curved letters such as a, e, n, and s tend to be slightly taller than the x-height for aesthetic purposes.
XML	see Extensible Markup Language.
XSL	see Extensible Stylesheet Language.
XSLT	see Extensible Stylesheet Language Transformations.

Y

None.

Z

None.