OpenType features are additional typographic options included in fonts using OpenType format.

For a single character, a font can contain multiple glyph variants (or alternates) alongside its default form. These can serve stylistic purposes (e.g., swashes, stylistic sets) or functional improvements (e.g., fractions, localized forms, contextual alternates).
Features can be activated in software that supports them (like design or word-processing apps) or through CSS on websites. Some common OpenType features include:
• Case-sensitive forms (case);
• Localized forms (locl);
• Small capitals or small caps (smcp);
• Tabular, proportional or oldstyle figures (tnum, pnum, onum);
• Stylistic sets (ss01-ss20);
• Superiors, inferiors (sups, sinf);
• Nominator, denominator, fractions (numr, dnum, frac).

You can find the full list of OpenType features here .

FONT ENGINEERING HINT

OpenType features are implemented in the font’s layout tables:
• GSUB (Glyph Substitution): replaces one glyph with another (e.g., default a with a swash variant).
• GPOS (Glyph Positioning): adjusts spacing and positioning between glyphs (e.g., kerning, mark placement).