A formal grammar provides an axiom schema for (or "generates") a "formal language", which is a (usually infinite) set of finite-length sequences of symbols that may be constructed by applying production rules to another sequence of symbols (which initially contains just the start symbol). A rule may be applied by replacing an occurrence of the symbols on its left-hand side with those that appear on its right-hand side. A sequence of rule applications is called a "derivation". Such a grammar defines the formal language: all words consisting solely of terminal symbols which can be reached by a derivation from the start symbol.