COMP as a functional head
Motivation for assuming a COMP node:
I. Embedded clauses are commonly introduced by means of a certain element, known as the complementizer (e.g. that).
II. Verbs that take sentential complements:
(a) commonly select a particular kind of complement:
(b) can place such specific requirements only on the heads of their complements, if we assume that the features of a phrase originate on its head.
III. We thus hypothesize the functional head COMP, which dominates the functional clause-type feature (or complementizers that realize this feature). This feature is referred to as [WH] and can have two values: [+WH] for interrogative (e.g. complementizer whether) and [-WH] for declarative (e.g. complementizer that).