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.^[2] The heads in the following phrases are in bold:

too slowly - Adverb phrase (AdvP)

very happy - Adjective phrase (AP)

the massive dinosaur - Noun phrase (NP)

at lunch - Preposition phrase (PP)

watch TV - Verb phrase (VP)

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.^[3] 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: