Prescriptive grammar is a philosophy or approach to grammar that concerns itself with the establishment of grammatical norms that can be used to define spoken or written language as either grammatically correct or grammatically incorrect. Under a prescriptive grammar approach, language rules are generally believed to change very little over time and allow for few exceptions. This philosophy of grammar contrasts with descriptive grammar, which is an approach to grammar that relies heavily on descriptions of how native speakers of a language who have achieved linguistic competence typically use language.
The the term prescriptive grammar may calls to mind an image of a stern grade school teacher who lays out, or prescribes, rules for the correct use of language. Some exacting language teachers insist that these rules must be adhered to at all times, even in casual speech. Sentences, clauses, and phrases that are acceptable on the playground may be disallowed in the classroom if they are deemed “incorrect.” This can occur in spite of their common use in the real world, where they are generally accepted without question.
Some prescribed language rules can be useful in writing composition classes but are regularly broken in typical language use. These types of prescribed rules tend to become well known, as they are frequently repeated after being broken and people often have a difficult time figuring out how to say what they want to say without breaking the rule. An example of a rule such as this is “never end a sentence with a preposition.” The problem is that there are many sentences that would be impossible to construct gracefully without a preposition at the end, such as “What did you step on?”
Prescriptive grammar contrasts with descriptive grammar. Within the realm of linguistics, language usage is believed to be dependent on an internalized grammar that allows members of a language community to produce sentences that can be readily understood by other members of the language community who have the same internalized grammar. Descriptive grammar can be broken down into several distinct areas of language use, which include phonology, morphology, syntax, and semantics.
The term prescriptive grammar can also be used to refer to a book containing a list of rules that have been laid out by a writer or group of writers considered experts in correct language use. Since language use changes over time, a prescriptive grammar published 25 or 50 years ago will often contain some rules that are outdated or simply no longer followed by the majority of the population.