Specifier

In X-bar theory in linguistics, specifiers, head words, and complements together form phrases. Specifiers differ from complements because they are not sisters of the head, but rather sisters of the phrase formed by the head and the complement. In English, some example of specifiers are words such as the, no, some, every, John's and my mother's which can precede noun phrases. Adverbial phrases can be preceded by degree words such as very, extremely, rather and quite.

These specifiers are so called because they further qualify the category of the head - in these examples nouns and adverbs - in the phrase.

For example:

• My friend likes [Jane Austen's novels] - Jane Austen's specifies novels in this noun phrase
• She is [quite certain of success] - quite specifies certain of success in the adverbial phrase quite certain of success

In recent transformational grammar, the term specifier is not normally used to refer to a type of word or phrase, but rather to a structural position provided by X-bar theory or some derivative thereof. In this usage, a phrase (usually a full XP, though in bare phrase structure it could in theory be an intermediate category) is said to occupy the specifier (SpecXP for short) of a head X.

In technical syntax terminology, specifier is the sister of X′ in the X-bar schema of phrase structure seen in the tree diagram below (where XP corresponds to X″):

Different form classes can occupy a specifier position, typically determiners and possessors in noun phrases (N″), and an auxiliary verb in a verb phrase (V″).