A rule in generative grammar of the form A → X, where A represents a syntactic category such as a sentence or a noun phrase and X a sequence of syntactic categories. The rule expresses an instruction to replace A by X. For example, a sentence (S) can be rewritten as a noun phrase (NP) plus a verb phrase (VP): S → NP + VP; and the verb phrase can be rewritten as a verb (V) plus a noun phrase (NP): VP → V + NP. Also called a phrase-structure rule. See also phrase marker.