First, prepositions will sometimes have other prepositional phrases as complements:
(10) The plane emerged from behind the cloud.
From and behind are both prepositions. But notice that from behind the cloud forms a single constituent. You can move it to the front of a cleft sentence:
(10a) From behind the cloud is where the plane emerged.
(2) Ken looked up her number.
(3) Ken looked up her dress.
A little scrutiny will show that up does not have the same function in both sentences. For example, while we can create a cleft sentence with up her dress, we can't do the same thing with up her number:
(2a) *Up her number is what Ken looked.
(3a) Up her dress is where Ken looked.
Also, we can move up to the end of the first sentence, but not the second:
(2b) Ken looked her number up.
(3b) *Ken looked her dress up.
We have already defined prepositions as a class of words that most commonly express relationships of space or time, or which mark syntactic functions.
Examples of Prepositions:
Spatial Relationship: behind the house
Temporal Relationship: after the party
Syntactic Function: the crux of the matter
Like other major word classes, prepositions are the heads of their own phrases. Prepositions are typically followed by a complement, called the object of the preposition. Most of the time, the object of the preposition is a noun phrase. In other words, the abstract phrase structure generally looks like this: