When we use a tree diagram format, we can think of it in two different ways. In one way, we can simply treat it as a static representation of the structure of the sentence shown at the bottom of the diagram. We could then propose that, for every single sentence in English, a tree diagram of this type could be drawn. An alternative view is to treat the tree diagram as a dynamic format, in the sense that it represents a way of generating not only that one sentence, but a very large number of other sentences with similar structures.

This second approach is very appealing because it would enable us to generate a very large number of sentences with what look like a very small number of rules. These rules are called phrase structure rules. As the name suggests, these rules state that the structure of a phrase of a specific type will consist of one or more constituents in a particular order. We can use phrase structure rules to present the information of the tree diagram in another format. That is, the information shown in the tree diagram on the left can be expressed in the phrase structure rule on the right.

According to this rule, “a noun phrase rewrites as an article followed by a noun.” The first rule in the following set of simple (and necessarily incomplete) phrase structure rules states that “a sentence rewrites as a noun phrase and a verb phrase.” The second rule states that “a noun phrase rewrites as either an article plus an optional adjective plus a noun, or a pronoun, or a proper noun.” The other rules follow a similar pattern.