Letters are a type of characters used in an alphabet to represent the sounds of a spoken language.

The same letter can have multiple sounds due to different pronunciation in different languages, and one sound can be transcribed with various letters or characters. Some letters can also bear diacritics to have more or less subtle variations of pronunciation in some languages.

Linguistically, a letter is an abstract symbol, but it can appear in different glyph forms, such as cases (uppercase or lowercase), stylistic variants, or contextual forms.

FONT ENGINEERING HINT

In a font file, a letter is encoded as a character (with a unique Unicode code point) but it may correspond to multiple glyphs. For example, an a can have a single or double storey form.
Some letters can also be combined into a single glyph to form a ligature (such as fi or æ).

These substitutions are handled in the gsub (Glyph Substitution) table, while the character-to-glyph mapping is defined in the cmap (Character To Glyph Index Mapping) table.