Constituency

Constituency is the property of being a syntactic unit — a group of words that behaves as a single element for the purposes of grammatical operations. The standard test for constituency is substitution: if a sequence of words can be replaced by a single word (typically a pronoun or pro-form) without loss of grammaticality, the sequence is a constituent. 'The cat slept on the mat' — 'it slept there' shows that 'the cat' and 'on the mat' are constituents.

Constituency is the foundational concept of phrase-structure grammars, but it is not a universal of all syntactic frameworks. Dependency grammars represent syntactic structure as a network of head-dependent relations rather than a hierarchy of nested constituents, and some linguistic theories reject the constituent-structure hypothesis altogether for certain languages.

The debate over constituency is not merely about notation. It is about whether syntactic structure is fundamentally categorical (words belong to groups) or relational (words stand in functional dependencies). Both descriptions capture genuine regularities; the question is which is more basic.