Functional Categories: DP, IP, and CP
Functional heads have specific properties which distinguish them from lexical categories.
(a) Functional heads carry grammatical properties rather than lexical/semantic properties:
INFL (or I°) bears finiteness and agreement features;
DET (or D°) bears features such as number, gender, definiteness, case (possessive);
COMP (or C°) bears clause-type features such as declarative or interrogative.
(b) Functional heads take one specific complement:
COMP takes an IP-complement;
INFL takes a VP-complement;
D° takes an NP-complement.
D as a functional head
INFL as a functional head
COMP as a functional head