The way linguists use the word ‘grammar’ differs from most common usages. Grammar is the knowledge speakers have about the units and rules of their language: rules for combining sounds into words called ‘phonology’, rules of word formation called ‘morphology’, rules for combining words and phrases into sentences called ‘syntax’, as well as the rules for assigning meaning called ‘semantics’. The grammar, together with a mental dictionary called a ‘lexicon’ that lists the words of the language, represents our linguistic competence. Every human being who speaks a language knows its grammar. When linguists wish to describe a language, they make explicit the rules of the grammar of the language that exist in the minds of its speakers. There will be some differences among speakers, but there must be shared knowledge too. The shared knowledge makes it possible to communicate through language. To the extent that the linguist’s description is a true model of the speakers’ linguistic capacity, it is a successful description of the grammar and of the language itself. Such a model is called a descriptive grammar.