In everyday speech, a phrase may refer to any group of words. In linguistics, a phrase is a group of words (or sometimes a single word) that form a constituent and so function as a single unit in the syntax of a sentence. A phrase is lower on the grammatical hierarchy than a clause.
Examine the following sentence:
- The house at the end of the street is red.
The words in bold form a phrase; together they act like a noun. This phrase can be further broken down; a prepositional phrase functioning as an adjective can be identified:
- at the end of the street
Further, a smaller prepositional phrase can be identified inside this greater prepositional phrase:
- of the street
And within the greater prepositional phrase, one can identify a noun phrase:
- the end of the street
Phrases can be identified by constituency tests such as proform substitution (=replacement). For instance, the prepositional phrase at the end of the street could be replaced by an adjective such as nearby: the nearby house or even the house nearby. The end of the street could also be replaced by another noun phrase, such as the crossroads to produce the house at the crossroads.
Heads and dependents 
Most phrases have an important word defining the type and linguistic features of the phrase. This word is the head of the phrase and gives its name to the phrase category. The heads in the following phrases are in bold:
The head can be distinguished from its dependents (the rest of the phrase other than the head) because the head of the phrase determines many of the grammatical features of the phrase as a whole. The examples just given show the five most commonly acknowledged types of phrases. Further phrase types can be assumed, although doing so is not common. For instance one might acknowledge subordinator phrases:
- before that happened - Subordinator phrase (SP)
This "phrase" is more commonly classified as a full subordinate clause and therefore many grammars would not label it as a phrase. If one follows the reasoning of heads and dependents, however, then subordinate clauses should indeed qualify as phrases. Most theories of syntax see most if not all phrases as having a head. Sometimes, however, non-headed phrases are acknowledged. If a phrase lacks a head, it is known as exocentric, whereas phrases with heads are endocentric.
Representing phrases 
Many theories of syntax and grammar represent sentence structure using trees. The trees provide schematic illustrations of how the words of sentences are grouped. These representations show the words, phrases, and at times clauses that make up sentences. Any word combination that corresponds to a complete subtree can be seen as a phrase. There are two competing principles for producing trees, constituency and dependency. Both of these principles are illustrated here using the example sentence from above. The constituency-based tree is on the left, and the dependency-based tree on the right:
The constituency-based tree on the left is associated with a traditional phrase structure grammar, and the tree on the right is one of a dependency grammar. The node labels in the trees (e.g. N, NP, V, VP) mark the syntactic category of the constituents. Both trees take a phrase to be any combination of words that corresponds to a complete subtree. In the constituency tree on the left, each phrasal node (marked with P) identifies a phrase; there are therefore 8 phrases in the constituency tree. In the dependency tree on the right, each node that dominates one or more other nodes corresponds to a phrase; there are therefore 5 (or 6 if the whole sentence is included) phrases in the dependency tree. What the trees and the numbers demonstrate is that theories of syntax differ in what they deem to qualify as a phrase. The constituency tree takes three word combinations to be phrases (house at the end of the street, end of the street, and is red) that the dependency tree does not judge to be phrases. Which of the two tree structures is more plausible can be determined in part by empirical considerations, such as those delivered by constituency tests.
Confusion: phrases in theories of syntax 
The common use of the term "phrase" is different from that employed by some phrase structure theories of syntax. The everyday understanding of the phrase is that it consists of two or more words, whereas depending on the theory of syntax that one employs, individual words may or may not qualify as phrases. The trees in the previous section, for instance, do not view individual words as phrases. Theories of syntax that employ X-bar theory, in contrast, will acknowledge many individual words as phrases. This practice is due to the fact that sentence structure is analyzed in terms of a universal schema, the X-bar schema, which sees each head as projecting at least three levels of structure: a minimal level, an intermediate level, and a maximal level. Thus an individual noun, such as Susan in Susan laughed, will project up to an intermediate level and a maximal level, which means that Susan qualifies as a phrase. This concept of the phrase is a source of confusion for students of syntax.
Many other theories of syntax do not employ the X-bar schema and are therefore less likely to encounter this confusion. For instance, dependency grammars do not acknowledge phrase structure in the manner associated with phrase structure grammars and therefore do not acknowledge individual words as phrases, a fact that is evident in the dependency grammar trees above and below.
The verb phrase (VP) as a source of controversy 
Most if not all theories of syntax acknowledge verb phrases (VPs), but they can diverge greatly in the types of verb phrases that they posit. Phrase structure grammars acknowledge both finite verb phrases and non-finite verb phrases as constituents. Dependency grammars, in contrast, acknowledge just non-finite verb phrases as constituents. The distinction is illustrated with the following examples:
- The Republicans may nominate Newt. - Finite VP in bold
- The Republicans may nominate Newt. - Non-finite VP in bold
The syntax trees of this sentence are next:
The constituency tree on the left shows the finite verb string may nominate Newt as a phrase (= constituent); it corresponds to VP1. In contrast, this same string is not shown as a phrase in the dependency tree on the right. Observe that both trees, however, take the non-finite VP string nominate Newt to be a phrase, since in both trees nominate Newt corresponds to a complete subtree.
Since there is disagreement concerning the status of finite VPs (whether they are constituents or not), empirical considerations are needed. Grammarians can (again) employ constituency tests to shed light on the controversy. Constituency tests are diagnostics for identifying the constituents of sentences and they are thus essential for identifying phrases. The results of most constituency tests do not support the existence of a finite VP constituent.
See also 
- Constituent (linguistics)
- Dependency grammar
- Head (linguistics)
- Phrase structure grammar
- Verb phrase
- Kroeger 2005:35
- Kroeger 2005:37
- For a good introduction and discussion of phrases and the tree structures that represent phrases, see Sobin (2011:29ff.).
- Finch (2000:112) sees a phrase consisting of two or more words; individual words do not count as phrases.
- Concerning the inability of most constituency tests to identify finite VP as a constituent, see Miller (2011:54f.) and Osborne (2011:323f.).
- Finch, G. 2000. Linguistic terms and concepts. New York: St. Martin's Press.
- Kroeger, Paul 2005. Analyzing grammar: An introduction. Cambridge University Press.
- Miller, J. 2011. A critical introduction to syntax. London: continuum.
- Osborne, Timothy, Michael Putnam, and Thomas Gross 2011. Bare phrase structure, label-less structures, and specifier-less syntax: Is Minimalism becoming a dependency grammar? The Linguistic Review 28: 315-364.
- Sobin, N. 2011. Syntactic analysis: The basics. Malden, MA: Wiley-Blackwell.
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