Gsub

GSUB stands for Glyph Substitution, a component of the OpenType font specification that defines how sequences of glyphs may be replaced with alternative glyphs during text shaping. Its purpose is to implement ligatures, contextual or stylistic alternates, and other substitutions without changing the underlying glyph architecture at render time.

In an OpenType font, GSUB is one of the layout tables. It organizes rules into lookups, which are grouped into features assigned to scripts and languages. The core data elements include a ScriptList, a FeatureList, and a LookupList. Each feature (for example liga, dlig, calt, hlig, salt) enables a set of lookups that can substitute one glyph for another, replace multiple glyphs with a ligature, or perform substitutions based on surrounding context.

Substitution mechanisms cover single substitutions (one glyph replaced by another), ligature substitutions (sequences replaced by a ligature glyph), and contextual or chained substitutions where the replacement depends on neighboring glyphs. The specific substitution rules reside in subtables referenced by the lookups.

Rendering engines such as HarfBuzz, DirectWrite, CoreText, and Pango consult the GSUB table during shaping to determine the appropriate glyphs for a given script, language, and font features. GSUB is commonly used in concert with GPOS, the Glyph Positioning table, to achieve advanced typography such as ligatures and alternate glyph forms.