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Letter case (or just case) is the distinction between the letters that are in larger upper case (also uppercase, capital letters, capitals, caps, large letters, or more formally majuscule) and smaller lower case (also lowercase, small letters, or more formally minuscule) in the written representation of certain languages. The writing systems that distinguish between the upper and lower case have two parallel sets of letters, with each letter in one set usually having an equivalent in the other set. The two case variants are alternative representations of the same letter: they have the same name and pronunciation and are treated identically when sorting in alphabetical order.

Letter case is generally applied in a mixed-case fashion, with both upper- and lower-case letters appearing in a given piece of text for legibility. The choice of case is often prescribed by the grammar of a language or by the conventions of a particular discipline. In orthography, the upper case is primarily reserved for special purposes, such as the first letter of a sentence or of a proper noun, which makes the lower case the more common variant in regular text.

In some contexts, it is conventional to use one case only. For example, engineering design drawings are typically labelled entirely in upper-case letters, which are easier to distinguish individually than the lower case when space restrictions require that the lettering be very small. In mathematics, on the other hand, letter case may indicate the relationship between objects, with upper-case letters often representing "superior" objects (e.g. X could be a set containing the generic member x).

The terms upper case and lower case may be written as two consecutive words, connected with a hyphen (upper-case and lower-case – particularly if they pre-modify another noun), or as a single word (uppercase and lowercase). These terms originated from the common layouts of the shallow drawers called type cases used to hold the movable type for letterpress printing. Traditionally, the capital letters were stored in a separate shallow tray or "case" that was located above the case that held the small letters.

Majuscule (/məˈdʒʌskjuːl/ or /ˈmædʒəskjuːl/), for palaeographers, is technically any script in which the letters have very few or very short ascenders and descenders, or none at all (for example, the majuscule scripts used in the Codex Vaticanus Graecus 1209, or the Book of Kells). By virtue of their visual impact, this made the term majuscule an apt descriptor for what much later came to be more commonly referred to as uppercase letters.

Minuscule refers to lower-case letters. The word is often spelled miniscule, by association with the unrelated word miniature and the prefix mini-. This has traditionally been regarded as a spelling mistake (since minuscule is derived from the word minus), but is now so common that some dictionaries tend to accept it as a nonstandard or variant spelling. Miniscule is still less likely, however, to be used in reference to lower-case letters.