Phrase
Functional categories

Some modern theories of syntax introduce functional categories in which the head of a phrase is a functional lexical item. Some functional heads in some languages are not pronounced, but are rather covert. For example, in order to explain certain syntactic patterns which correlate with the speech act a sentence performs, some researchers have posited force phrases (ForceP), whose heads are not pronounced in many languages including English. Similarly, many frameworks assume that covert determiners are present in bare noun phrases such as proper names.

Another type is the inflectional phrase, where (for example) a finite verb phrase is taken to be the complement of a functional, possibly covert head (denoted INFL) which is supposed to encode the requirements for the verb to inflect – for agreement with its subject (which is the specifier of INFL), for tense and aspect, etc. If these factors are treated separately, then more specific categories may be considered: tense phrase (TP), where the verb phrase is the complement of an abstract "tense" element; aspect phrase; agreement phrase and so on.

Further examples of such proposed categories include topic phrase and focus phrase, which are argued to be headed by elements that encode the need for a constituent of the sentence to be marked as the topic or focus.
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