Letter case is the distinction between the letters that are in larger uppercase or capitals and smaller lowercase in the written representation of certain languages. The writing systems that distinguish between the upper- and lowercase have two parallel sets of letters: each in the majuscule set has a counterpart in the minuscule set. Some counterpart letters have the same shape, and differ only in size, but for others the shapes are different. The two case variants are alternative representations of the same letter: they have the same name and pronunciation and are typically treated identically when sorting in alphabetical order.

A writing system comprises a particular set of symbols, called a script, as well as the rules by which the script represents a particular language. Writing systems can generally be classified according to how symbols function according to these rules, with the most common types being alphabets, syllabaries, and logographies. Alphabets use symbols called letters that correspond to spoken phonemes. Abjads generally only have letters for consonants, while pure alphabets have letters for both consonants and vowels. Abugidas use characters that correspond to consonant–vowel pairs. Syllabaries use symbols called syllabograms to represent syllables or moras. Logographies use characters that represent semantic units, such as words or morphemes.