Publisher Summary This chapter describes several classes of sentence-generating devices that are closely related, in various ways, to the grammars of both natural languages and artificial languages of various kinds. Language means simply a set of strings in some finite set V of symbols called the vocabulary of the language. Grammar essentially means a set of rules that give a recursive enumeration of the strings belonging to the language. It is noted that for a class of grammars to have linguistic interest, there must be a procedure that assigns to any pair (σ, G), where σ is a string and G a grammar of this class. In particular, the structural description indicates that the string σ is a well-formed sentence of the language L (G) generated by G. However, the chapter concerns with only one aspect of the structural description of a sentence—namely, its subdivision into phrases belonging to various categories.