A term used in Linguistics, and especially in generative grammar, to refer to a formal statement of correspondence between linguistic elements or structures. In the classical account, a grammar is seen as a set of rewrite rules which will generate all and only the grammatical sentences of a language.The rules may be subclassified in terms of the components of the grammar in which they appear (e.g. ‘phonological rules’, ‘syntactic rules’, ‘lexical rules’). In Syntactic Structures (1957), a distinction was made between optional and obligatory rules. Other types of rule commonly cited include recursive, global, movement, readjustment, variable, lexical insertion and lexical redundancy rules. The linguistic sense thus contrasts with the traditional use of the term, where rules are recommendations for correct usage. No prescriptive or proscriptive implication is present in the linguistic sense of ‘rule’.