letter

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).

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