Glyph substitution is the process that automatically replaces one glyph with another based on specific rules or features existing in the font. It enables advanced typographic behavior within the font, such as contextual, stylistic, or language-specific variations (e.g. small caps, ligatures, alternate forms).

This mechanism is a cornerstone of the OpenType format, as it allows a single font file to contain multiple visual variations and apply them automatically depending on the context, language, or user’s choice. Without it, a font file would function more like with earlier formats: just a container for static outlines.

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In OpenType fonts, glyph substitution is handled by the GSUB (Glyph Substitution) table. It defines which glyphs can replace others under specific conditions. Substitutions can be triggered automatically by an application, or manually if a user activates its OpenType features.