The head of an expression consisting of a °noun and its °modifiers is the noun; the head of an expression consisting of a °verb and its °arguments and °adjuncts is the verb; and the head of an expression consisting of an °adjective and its °degree qualifiers is the adjective.
The "head" notion is typically defined in a more general way, so that any kind of phrase (or even a word) can have a head. However, it is unclear how "phrase" can be defined, and how the head of any phrase can be identified, and there is no consensus (see
head (CXN) = the most contentful word that most closely denotes the same function as the phrase (or clause) as a whole. Example: the head of the phrase an ancient watch is watch. (Section 2.2.1)