combine words into phrases and phrases into sentences.

describe the relationship between the meaning of a particular group of words and the arrangement of those words.

specify the grammatical relations of a sentence, such as subject and direct object.

Det—N—V—Det—N

The natural groupings or parts of a sentence are called constituents.

A family of expressions that can substitute for one another without loss of
grammaticality is called a syntactic category.

NOUN, VERB, PREPOSITION, ADJECTIVE, ADVERB

Phrase structure rules instantiate the principles of the X-bar schema and can be used as a guide for building PS trees

To account for the potentially limitless
number of adjectives we need a recursive rule—one that repeats itself—on N̅:

What Heads the Sentence