OpenTypefeatures

OpenType features are a mechanism in the OpenType font format that allows a font to substitute glyphs and adjust their positioning based on language, script, or typographic context. They are implemented in the font through GSUB (glyph substitution) and GPOS (glyph positioning) tables and are applied by text layout engines in operating systems, applications, and web browsers. Features are identified by tags and can be activated selectively to enable advanced typography without changing the underlying glyph shapes.

Common features include ligatures, substitutions, and stylistic variants. Standard ligatures are enabled by liga, which can replace letter pairs with a single ligature glyph (for example fi or fl). Discretionary ligatures (dlig) provide additional alternatives, while contextual alternating (calt) and contextual ligatures (clig) adjust glyphs based on surrounding characters. Stylistic alternatives (salt) and stylistic sets (ss01–ss20) allow fonts to offer multiple look variations. Other frequently used features cover small capitals (smcp), small capitals from capitals (c2sc), and numeral styles such as oldstyle figures (onum), lining figures (lnum), tabular figures (tnum), and proportional figures (pnum). Additional substitutions include ordinal forms (ordn), fractions ( frac), subscripts (subs) and superscripts (sups). Some fonts also provide historical ligatures (hlig) and other language- or font-specific variants.

Implementation and usage vary by software and platform. Enabling OpenType features can be done via UI in desktop and web applications, or through CSS on the web (for example, font-feature-settings or font-variant-ligatures). Not all fonts expose every feature, and the exact substitutions depend on the font’s design and the language context.

OpenType features have been part of the OpenType specification since its early versions, enabling programmable typography across platforms and applications.