Heads and Phrases

Two of the most foundational terms in syntax are the head and the phrase. A head is the morpheme that gives the lexical or grammatical category of the whole phrase. A phrase is a group of morphemes that acts as a unit and whose category is determined by its head. These terms are, in a sense, arbitrary labels, but they let us talk about structure in a systematic way.

An example

Take a sentence used repeatedly as an illustration: Ezra gave his stuffed animal to a child at 3 p.m. on Wednesday. The string a child is a phrase — specifically what is later called a determiner phrase. Here a is the determiner and child is the noun. Because the head of a child is the determiner a, the whole unit counts as a determiner phrase. The head names the phrase.

The same logic applies to verbs. A verb phrase has a verb as its head; that is simply what a verb phrase is by definition. In this sentence the verb phrase is gave his stuffed animal to a child. Notice that the phrase includes both the head and the arguments of the head: the verb give requires certain arguments, and together the head and its arguments make up the whole phrase.

Why phrases matter

The reason phrases are worth identifying is interpretive: morphemes that occur in the same phrase are interpreted together — they are, in effect, closer to one another. The whole point of drawing syntactic trees is to display this structure and to show which morphemes belong together.

Consider his stuffed animal, which forms one phrase, alongside a child, which forms another. Within a child, the determiner a is specifying child — clarifying how many and what kind, as opposed to the child. Roughly, a child is not previously known while the child is. So a affects the meaning of child, and within his stuffed animal, his affects stuffed animal. Crucially, his does not affect child. Dividing a sentence into phrases is how we make visible which words are interpreted together and which are not.