The constituent that we will see most is the phrase. A phrase consists of a single main word, called the head of the phrase, and other words that modify or give grammatical information about the head. These other words in the phrase are called the phrase's attributes. Informally, we might say that the head word is the main idea of the phrase.

(2) Russia's proposal at the conference

The phrase in example (2) is talking about a kind of proposal. Russia's and at the conference tell us what specific proposal we're talking about. Proposal, therefore is the head word.[1]

The lexical category of the phrase's head gives its name to phrase. Thus a noun is the head of a noun phrase (abbreviated NP), a verb the head of a verb phrase (VP), and so forth. Since proposal is a noun, (2) is a noun phrase.

Other Examples:

(3a)	baked him a cake	Verb Phrase (VP)
(3b)	fond of pecans	Adjective Phrase (AP)
(3c)	very quickly	Adverb Phrase (AdvP)
(3d)	to the lighthouse	Prepositional Phrase (PP)

 
Apart from simply being a convenient way to name phrases, the relationship between the head word and the phrase type captures a significant fact of syntax: the category of the head word plays an important role in determining where in the sentence the phrase can go, as well as a variety of grammatical rules such as agreement between subject and verb.

(4) {The [contract] between the boards of the two companies} [was] nullified by regulators.

For example, in sentence (4), contract is the head word of the NP which is the subject. The whole subject, therefore is singular, and agrees with the verb was, despite the two plural nouns (boards and companies) which are closer to the verb in terms of linear order, but which are actually buried in prepositional phrases.[2]

This example also illustrates another important point: phrase structure is hierarchical. That is, phrases can nest within phrases to any level of complexity. Thus the subject of (4), "the agreement between the boards of the two companies," contains two prepositional phrases, each of which itself contains a noun phrase. We can show this relationship in a diagram:

Take a moment to study this diagram. We will refine it later with additional details, but it's important that you recognize what information it's trying to communicate. It shows that the whole noun phrase contains three parts: a determiner, the, the head noun, agreement, and a prepositional phrase, between the boards of the two companies. In turn, that prepositional phrase consists of its head word, between and a noun phrase, the boards of the two companies. That noun phrase contains yet another prepositional phrase, of the two companies, which contains its own noun phrase, the two companies. That's what we mean when we say that phrase structure is hierarchical: one phrase can contain another phrase inside it.

Viewed this way, even the most elaborate sentence can always be broken down into a handful of relatively simple patterns that repeat over and over.

One final note on phrases: in ordinary, non-technical usage, the word phrase means "more than one word." Thus you will sometimes encounter books that use the expression "word or phrase" to explain concepts like the subject. As we have defined phrases here, however, that expression is redundant. Because the attributes of a phrase are often optional, it is possible to have a phrase that consists of a single word.

(5) Computers intimidate many people.
(6) The young man was naïve.

In sentence (5), computers is a noun in a phrase with no attributes. It is a noun phrase all by itself. In sentence (6), naïve is a one-word adjective phrase. Treating these constituents as phrases, and not just individual words, allows us to account for many aspects of grammar in a simpler and more consistent fashion than if we treated them differently.

Notes

[1] This semantic test works reasonably well for prototypical cases, but be careful. There are many cases where the idea of the phrase won't really match the structural head. In other words, this notional definition of the head is meant to get you started with the easy cases, but it's only a rough guide. As you look at the examples that follow, pay attention to the structural patterns first, and meaning second.

[2] We are, of course, speaking of Standard English when we refer to subject-verb agreement rules. Some people do from time to time operate by a principle of attraction, making the verb agree with the nearest noun rather than the head noun of the whole phrase.