Generative grammars can be described and compared with the aid of the Chomsky hierarchy (proposed by Chomsky in the 1950s). This sets out a series of types of formal grammars with increasing expressive power. Among the simplest types are the regular grammars (type 3); Chomsky claims that these are not adequate as models for human language, because of the allowance of the center-embedding of strings within strings, in all natural human languages.