We have noted how verb phrases have a verb as the head, noun phrases have a noun (or pronoun), and adverb phrases and adjective phrases have an adverb and an adjective as the head respectively. Prepositional phrases (PrepP) do not follow this pattern.
They are unusual in that they do not have a head. Instead, they have two compulsory elements, a preposition and a noun phrase. Consider the following.
The prepositional phrase is only complete if both the preposition and noun phrase are combined. That is to say, neither the cat sat on, which omits the noun phrase, nor the cat sat the mat, which omits the preposition, would be correct. Consider a further example.
In this example we see that the two compulsory elements, a preposition and a noun phrase, are in evidence. Again, one without the other would make the utterance nonsensical or incomplete, i.e. neither the big bird sang in, omitting the noun phrase, nor the big bird sang the leafy tree, omitting the preposition, would be grammatical. Now, because prepositional phrases have no head they cannot be pre-modified. However, prepositional phrases themselves can be used as post-modifiers of noun phrases, i.e. they may follow the head noun in such phrases.
Post-modification of noun phrases
Consider the following example of a noun phrase in which a prepositional phrase is used to post-modify the head noun.
The noun phrase is the coat of many colors and the head noun is coat. This head noun is pre-modified by the identifier the. In addition, it is post-modified by the prepositional phrase of many colors. It is a simple matter to determine if this complex structure is functioning as a complete noun phrase, as it is always possible to substitute a pronoun for a whole noun phrase. So, in the utterance the coat of many colors was beautiful a pronoun may be substituted for the noun phrase to produce the modified utterance it was beautiful. Here is a further example of a prepositional phrase being used to post-modify a noun phrase.
The noun phrase is the women in my beautiful life, the head noun being women. As before, it is pre-modified by the identifier the and post-modified by a prepositional phrase. The prepositional phrase in my beautiful life consists, as it should, of both a preposition (in) and a noun phrase (my beautiful life).
Not only can prepositional phrases be used to post-modify noun phrases, however, they can also be used to post-modify adjective phrases.
Post-modification of adjective phrases
Consider the following example that demonstrates the use of a prepositional phrase to post-modify an adjective phrase.
The adjective phrase is confident of her expertise and the head adjective is confident. It is this head adjective that is post-modified by the prepositional phrase of her expertise. This prepositional phrase consists of the compulsory preposition (of) and noun phrase (her expertise). A further example of post-modification of an adjective phrase with a prepositional phrase is shown below.
In this example, the adjective phrase is extremely happy in their home. The head adjective is happy and we see that this has been both pre- and post-modified. It is pre-modified by the intensifying adverb extremely and post-modified by the prepositional phrase in their home. This prepositional phrase, once more, consists of the two compulsory components of a preposition (in) and a noun phrase (their home).
There are actually several other ways in which noun phrases and adjective phrases may be post-modified but space does not permit a more detailed discussion at present. However, the examples provided should be sufficient to demonstrate how words are combined into the larger phrasal units of language.