“Descriptive and Prescriptive Grammar: Contrasting terms in linguistics. A descriptive grammar is an account of a language that seeks to describe how it is used objectively, accurately, systematically, and comprehensively. A prescriptive grammar is an account of a language that sets out rules (prescriptions) for how it should be used and for what it should not be used (proscriptions), based on norms derived from a particular model of grammar. For English, such a grammar may prescribe I as in It is I and proscribe me as in It’s me. It may proscribe like used as a conjunction, as in He behaved like he was in charge, prescribing instead He behaved as if he were in charge. Prescriptive grammars have been criticized for not taking account of language change and stylistic variation, and for imposing the norms of some groups on all users of a language. They have been discussed by linguists as exemplifying specific attitudes to language and usage. Traditional grammar books have often, however, combined description and prescription. Since the late 1950s, it has become common in linguistics to contrast descriptive grammars with generative grammars. The former involve a description of linguistic structures, usually based on utterance elicited from native-speaking informants. The latter, introduced by Chomsky, concentrate on providing an explicit account of an ideal native speaker’s knowledge of language (competence) rather than a description of samples (performance). Chomsky argued that generative grammars are more valuable, since they capture the creative aspect of human linguistic ability. Linguists generally regard both approaches as complementary.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.