Constituent (grammar)

In syntactic analysis, a constituent is a word or a group of words that 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 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.

Constituency tests are diagnostics used to identify the constituent structure of sentences. There are numerous constituency tests applied to English sentences, such as:

These tests are rough-and-ready tools which grammarians employ to reveal clues about syntactic structure. A word of caution is warranted when employing these tests, since they often deliver contradictory results. 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. Failing to pass a single test does not mean that the unit is not a constituent, and conversely, passing a single test does not mean necessarily that the unit is a constituent. It is best to apply as many tests as possible to a given unit in order to prove or to rule out its status as a constituent.

Topicalization involves moving the test sequence to the front of the sentence. It is a simple movement operation:

Clefting involves placing a sequence of words X within the structure beginning with It is/was: It was X that...

Pseudoclefting (also preposing) is similar to clefting in that it puts emphasis on a certain phrase in a sentence. It involves inserting a sequence of words before is/are what or is/are who:

Pro-form substitution, or replacement, involves replacing the test constituent with the appropriate pro-form (e.g. pronoun). Substitution normally involves using a definite pro-form like it, he, there, here, etc. in place of a phrase or a clause. If such a change yields a grammatical sentence where the general structure has not been altered, then the test sequence is a constituent:

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