Phrase structure grammar was introduced by Noam Chomsky in the 1950s as an evolution of Immediate Constituent Analysis. Phrase structure grammars use rewrite rules to break down linguistic units like sentences into constituent parts such as noun phrases and verb phrases. The rewrite rules represent how morphemes are organized into words, words into phrases, and phrases into sentences. Phrase structure grammars have an advantage over older models as they incorporate labels into rewrite rules and allow rules to be applied sequentially to generate terminal strings.