The natural mechanism for generating Phrase-markers [...] is a system of 'rewriting rules'. A rewriting rule is a rule of the form (4) A → Z/X − Y where 'X' and 'Y' are (possibly null) strings of symbols, 'A' is a single category symbol, and 'Z' is a nonnull string of symbols. This rule is interpreted as asserting that the category 'A' is realized as the string 'Z' when it is in the environment consisting of 'X' to the left and 'Y' to the right. Application of the rewriting rule (4) to a string …'XAY'... converts it to the string …'XZY'....
- Chomsky (1969), a pag.66