A grammar specifies the legal syntax of a language. The kind of grammar most commonly used in computer language processing is a context-free grammar. A grammar specifies a set of productions; non-terminal symbols (phrase names or parts of speech) are enclosed in angle brackets. Each production specifies how a nonterminal symbol may be replaced by a string of terminal or nonterminal symbols, e.g., a Sentence is composed of a Noun Phrase followed by a Verb Phrase.
< S> --> < NP> < VP> < NP> --> < ART> < ADJ> < NOUN> < NP> --> < ART> < NOUN> < NP> --> < ART> < NOUN> < PP> < VP> --> < VERB> < NP> < VP> --> < VERB> < NP> < PP> < PP> --> < PREP> < NP> < ART> --> A | AN | THE < NOUN> --> BOY | DOG | LEG | PORCH < ADJ> --> BIG < VERB> --> BIT < PREP> --> ON