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The application of layout features to one or more characters may change the positioning, or substitute a different glyph.

For any character, there is a default glyph and positioning behavior. OpenType layout features can be used to position or substitute glyphs.

One glyph can also represent multiple characters, as in the case of the “ffi” ligature, which corresponds to a sequence of three characters: f, f and i. One character may correspond to several glyphs the lowercase “a,” a small cap “a” and an alternate swash lowercase “a” are all the same character, but they are three separate glyphs. Glyphs are the specific forms that those characters can take. Characters are the code points assigned by the Unicode standard, which represent the smallest semantic units of language, such as letters. Central to a discussion of OpenType feature support lies the distinction between characters and glyphs.