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 rhsn the expansion. Such rules are context-free rules because the expansion is unconditional — the environment of the constituent to be expanded is irrelevant.