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Developing fonts > Specifications

Feature tags

Features provide information about how to use the glyphs in a font to render a script or language. For example, an Arabic font might have a feature for substituting initial glyph forms, and a Kanji font might have a feature for positioning glyphs vertically. All OpenType Layout features define data for glyph substitution, glyph positioning, or both.

Each OpenType Layout feature has a feature tag that identifies its typographic function and effects. By examining a feature's tag, a text-processing client can determine what a feature does and decide whether to implement it. All tags are 4-byte character strings composed of a limited set of ASCII characters in the 0x20-0x7E range. Microsoft-registered feature tags use four lowercase letters. For instance, the "mark" feature manages the placement of diacritical marks, and the "swsh" feature renders swash glyphs.

A feature definition may not provide all the information required to properly implement glyph substitution or positioning actions. In many cases, a text-processing client may need to supply additional data. For example, the function of the "init" feature is to provide initial glyph forms. Nothing in the feature's lookup tables indicates when or where to apply this feature during text processing. To correctly use the "init" feature in Arabic text where initial glyph forms appear at the beginning of words, text-processing clients must be able to identify the first glyph position in each word before making the glyph substitution. In all cases, the text-processing client is responsible for applying, combining, and arbitrating among features and rendering the result.

The tag space defined by tags consisting of four uppercase letters (A-Z) with no punctuation, spaces, or numbers, is reserved as a vendor space. Font vendors may use such tags to identify private features. For example, the feature tag "PKRN" might designate a private feature that may be used to kern punctuation marks. Microsoft does not guarantee the compatibility or usability of private features, and it cannot ensure that two font vendors will not choose the same tag for a private feature.

This version of the Tag Registry describes all the OpenType Layout features Microsoft has developed to date. It also includes details that identify the lookups that Microsoft uses to implement each feature. Lookup information is provided for reference purposes only; the set of lookups used to implement a feature will vary across system platforms, applications, fonts, and font developers.

Registered features

Feature TagFriendly Name
aaltAccess All Alternates
abvfAbove-Base Forms
abvmAbove-Base Mark Positioning
abvsAbove-Base Substitutions
afrcAlternative Fractions
blwfBelow-Base Forms
blwmBelow-Base Mark Positioning
blwsBelow-Base Substitutions
c2pcPetite Capitals From Capitals
c2scSmall Capitals From Capitals
caltContextual Alternates
caseCase-Sensitive Forms
ccmpGlyph Composition/Decomposition
cligContextual Ligatures
cjctConjunct Forms
cpspCapital Spacing
cswhContextual Swash
cursCursive Positioning
dfltDefault Processing
dligDiscretionary Ligatures
exptExpert Forms
faltFinal glyph Alternates
fin2Terminal Forms #2
fin3Terminal Forms #3
finaTerminal Forms
fwidFull Width
halfHalf Forms
halnHalant Forms
haltAlternate Half Width
histHistorical Forms
hknaHorizontal Kana Alternates
hligHistorical Ligatures
hojoHojo Kanji Forms (JIS x 212-1990 Kanji Forms)
hwidHalf Width
initInitial Forms
isolIsolated Forms
jaltJustification Alternatives
jp78JIS78 Forms
jp83JIS83 Forms
jp90JIS90 Forms
jp04JIS2004 Forms
lfbdLeft Bounds
ligaStandard Ligatures
ljmoLeading Jamo Forms
lnumLining Figures
loclLocalized Forms
markMark Positioning
med2Medial Forms #2
mediMedial Forms
mgrkMathematical Greek
mkmkMark to Mark Positioning
msetMark Positioning via Substitution
naltAlternate Annotation Forms
nlckNLC Kanji Forms
nuktNukta Forms
onumOld Style Figures
opbdOptical Bounds
paltProportional Alternate Width
pcapPetite Capitals
pnumProportional Figures
prefPre-base Forms
presPre-base Substitutions
pstfPost-base Forms
pstsPost-base Substitutions
pwidProportional Widths
qwidQuarter Widths
rkrfRakar Forms
rligRequired Ligatures
rphfReph Form
rtbdRight Bounds
rtlaRight-To-Left Alternates
rubyRuby Notation Forms
saltStylistic Alternates
sinfScientific Inferiors
sizeOptical Size
smcpSmall Capitals
smplSimplified Forms
ss01Sylistic Set 1
ss02Sylistic Set 2
ss03Sylistic Set 3
ss04Sylistic Set 4
ss05Sylistic Set 5
ss06Sylistic Set 6
ss07Sylistic Set 7
ss08Sylistic Set 8
ss09Sylistic Set 9
ss10Sylistic Set 10
ss11Sylistic Set 11
ss12Sylistic Set 12
ss13Sylistic Set 13
ss14Sylistic Set 14
ss15Sylistic Set 15
ss16Sylistic Set 16
ss17Sylistic Set 17
ss18Sylistic Set 18
ss19Sylistic Set 19
ss20Sylistic Set 20
tjmoTrailing Jamo Forms
tnamTraditional Name Forms
tnumTabular Figures
tradTraditional Forms
twidThird Widths
valtAlternate Vertical Metrics
vatuVattu Variants
vertVertical Writing
vhalAlternate Vertical Half Metrics
vjmoVowel Jamo Forms
vknaVertical Kana Alternates
vkrnVertical Kerning
vpalProportional Alternate Vertical Metrics
vrt2Vertical Rotation
zeroSlashed Zero

To register features

Microsoft encourages font developers to use 'registered' feature tags when implementing registered features. However, font developers also may define and register their own features.

Microsoft welcomes nominations for new features and feature tags to register. To qualify for registration, a feature must have a single function that is clearly identified by its tag. The function of the feature should be defined at the lowest useful level and must be distinctly different from the functions of currently registered features. When font developers register feature tags and functions with Microsoft, they do not have to supply implementation details.

Microsoft reserves the right to officially assign feature tags in the Microsoft Tag Registry. Although Microsoft has reserved the feature and feature tag definitions listed in this registry, Microsoft fonts do not necessarily contain all of the features.

this page was last updated 30 March 2005

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