A preposition relates one unit in the sentence to something else in the sentence. Prepositions often express relations of space or time, or they mark various grammatical roles. Words like in, to, over, and through are prepositions. As their name implies, they precede something, usually a noun phrase. The phrase that follows a preposition is called the object of a preposition.

(21) in [the yard]
(22) throughout [the ages]

Prepositions are slightly different from the categories we have already examined. They often have distinct meanings of their own, but many prepositions play a more purely functional. Prepositions form a small, relatively closed set of words. There are only a few hundred prepositions in English, as opposed to tens of thousands of nouns, verbs, adjectives, or adverbs. It's easy to invent new nouns, verbs, adjectives, or adverbs. New prepositions, however, are added to the language only rarely.

Prepositions do not have inflectional endings, so we cannot apply morphological tests to prepositions. However, like adjectives, many prepositions are gradable. These prepositions can be preceded by degree words such as right or straight:

(23) She walked right into the wall.

Not every preposition is gradable, however. Of is a preposition, but it cannot be modified by right/straight.

(24) *The relaxed days right of summer were my favorite.

The ungradable prepositions have what are called grammaticalized uses. In other words, the preposition's meaning is not distinguishable from the grammatical construction in which it occurs. For example, compare the use of by in the following sentences:

(25a) His blind date stood by the fountain.
(26a) The report was completed by a committee of experts.

In (25a), by has an identifiable spatial meaning. This use is not grammaticalized. In (26), however, by has no spatial meaning. Indeed, it's hard to say what independent meaning it has. Its function is grammatical: it specifies the following noun phrase (a committee of experts) as the actor in the sentence. Notice that (25a) is gradable, but (26a) is not:

(25b) His blind date stood right by the fountain.
(26b) *The report was completed right by a committee of experts.