Language has often been characterized as a systematic correlation between certain types of gestures and meaning

1.3 Phrases as the basic element of syntax

In the passive version of the sentence (3) The shaft was brushed against his shoulder by a passing cab the unit the shaft serves as subject, and the unit the passing cab is the object of the preposition by. The constituent composed of a noun and an article is called a noun phrase [NP], e.g. by the teacher; NPs can be very complex. Here is a list of some examples of NP:

the girl beautiful weather

this boy those sunny days
a dog stupid question
that large bicycle nice try
women the Pacific Ocean
elderly men brilliant student
David this year
Queensland judgment day

They water rat

What structure do noun phrases have in English? Based on the noun phrases listed above (there are more complicated ones), a noun phrase seems to consist of a determinative followed by a noun, or a determinative followed by an adjective followed by a noun, or just a noun, or an adjective followed by a noun.

We can represent these structures using what are called phrase structure rules, like:

NP → Detv N

This rule says that a noun phrase (NP) “goes to” (arrow) a determinative (Detv) followed by a noun (N). We could thus separately list the rules that we would need to cover all the structures:

NP → Detv N

NP → Detv Adj N
NP → N
NP → Adj N

In fact, there’s a simpler way to write all of these different forms with a single rule. There is a convention in writing phrase structure rules so that if something is in parentheses, it can either be there or not. So we could rewrite our rules just as:

NP → (Detv) (Adj) N

This rule says that a noun phrase consists of a noun, possibly preceded by a determinative.

The preposition by and the NP following it in the sentence also form a constituent in this sentence (by a passing cab); it is called a prepositional phrase [PP].