we say that [hats] too is not just a noun, but a noun phrase

(11) a the hats

      b the blue hats

      c the blue hats on the shelf

 

In each of these strings of words the central element is the noun hats.

This central element in a phrase is referred to as its Head.

Heads function as the central elements of phrases.

Noun phrases can become very long, indefinitely long in fact, if we keep adding elements.

But they can also consist of only a Head.

 

(12) [ Hats ] have always been fashionable.

 

Why can we label hats as a noun phrase? 

Distributionally, it is because the slot in (12) in which hats occurs can be filled by a larger NP (e.g. blue hats).

 

(13) a These hats have always been fashionable.

      b Hats that you buy at Harrods have always been fashionable.

 

The sentence-initial (Subject) position in which these NPs occur is a typical noun phrase position.

On the grounds that we can expand hats in (12) into a larger string of words which is clearly an NP, and because it occurs in a typical NP position, we say that hats too is not just a noun, but a noun phrase.