Let us consider an example of a context-free grammar for generating very simple English sentences. It uses the initial symbol S of a sentence to be generated and several other non-terminal symbols: the noun phrase symbol NP, verb phrase symbol VP, noun symbol N, verb symbol V, and determinant symbol D. All these non-terminal symbols are interpreted as grammatical categories.
Several production rules for replacement of a non-terminal symbol with a string of several other non-terminal symbols are used as the nucleus of any generative grammar. In our simple case, let the set of the rules be the following: