Letter case is often prescribed by the grammar of a language or by the conventions of a particular discipline. In orthography, the uppercase is primarily reserved for special purposes, such as the first letter of a sentence or of a proper noun, which makes the lowercase the more common variant in text. In mathematics, letter case may indicate the relationship between objects with uppercase letters often representing "superior" objects (e.g. X could be a set containing the generic member x). Engineering design drawings are typically labelled entirely in upper-case letters, which are easier to distinguish than lowercase, especially when space restrictions require that the lettering be small.