Constituent (linguistics)

In syntactic analysis, a constituent is a word or a group of words that function as a single unit within a hierarchical structure. The constituent structure of sentences is identified using tests for constituents. These tests apply to a portion of a sentence, and the results provide evidence about the constituent structure of the sentence. Many constituents are phrases. A phrase is a sequence of one or more words (in some theories two or more) 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. 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. Tests for constituents are diagnostics used to identify sentence structure. There are numerous tests for constituents that are commonly used to identify the constituents of English sentences. 15 of the most commonly used tests are listed next: 1) coordination (conjunction), 2) pro-form substitution (replacement), 3) topicalization (fronting), 4) do-so-substitution, 5) one-substitution, 6) answer ellipsis (question test), 7) clefting, 8) VP-ellipsis, 9) pseudoclefting, 10) passivization, 11) omission (deletion), 12) intrusion, 13) wh-fronting, 14) general substitution, 15) right node raising (RNR). The order in which these 15 tests are listed here corresponds to the frequency of use, coordination being the most frequently used of the 15 tests and RNR being the least frequently used. A general word of caution is warranted when employing these tests, since they often deliver contradictory results. The tests are merely rough-and-ready tools that grammarians employ to reveal clues about syntactic structure. Some syntacticians even arrange the tests on a scale of reliability, with less-reliable tests treated as useful to confirm constituency though not sufficient on their own.