A grammar specifies the legal syntax of a language. The kind of grammar most often 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