Context-Free Grammar

Abstract

A context-free grammar is a collection of context-free phrase structure rules. Each such rule names a constituent type and specifies a possible expansion thereof. The standard notation is:

$$ {\rm{lhs}}\,\, \to \,\,{\rm{rh}}{{\rm{s}}_{\rm{1}}}\,\,.\,\,.\,\,.\,\,{\rm{rh}}{{\rm{s}}_{\rm{n}}} $$

where lhs names the constituent, and rhs₁ through rhs_n the expansion. Such rules are context-free rules because the expansion is unconditional — the environment of the constituent to be expanded is irrelevant.