In syntactic analysis, a constituent is a word or a group of words which function(s) as a single unit within a hierarchical structure. The analysis of constituent structure is associated mainly with phrase structure grammars, although dependency grammars also allow sentence structure to be broken down into constituent parts. The constituent structure of sentences is identified using constituency tests. These tests manipulate some portion of a sentence and based on the result, clues are delivered about the immediate constituent structure of the sentence. Many constituents are phrases. A phrase is a sequence of two or more words built around a head lexical item and working as a unit within a sentence. A word sequence is shown to be a phrase/constituent if it exhibits one or more of the behaviors discussed below.