In X-bar theory, what is considered the core element of any phrase?

In X-bar theory, what is considered the core element of any phrase?

Explanation

In X-bar theory, a concept from generative grammar, the core element is the head, which determines the nature of the phrase

The head is the central word of a phrase (e.g., a noun in a noun phrase). 

It determines the syntactic type of the phrase (Noun Phrase, Verb Phrase, etc.)

"The big book about history"

● Head: "book" (noun)

● Specifier: "The"

● Modifier: "big"

● Complement: "about history