phrases function attributively to give additional information about an adjacent noun, the modifier adverbial phrases illustrated in (5) to (7) function as secondary predicates that give additional temporal information about the sentence. (5) I'll go to bed in an hour. (6) We arrived three hours late. (7) Later in the day the situation had resolved. Complement adverbial phrases Complement adverbial phrases are much less common than their modifier counterparts. Adverbial phrases functioning as complements usually arise when an adverb licenses a complement as a selectional requirement. Nearly all of these complements license an adjoining prepositional phrase. Below are a few examples of complement adverbial phrases. (8) Purchase of State vehicles is handled [similarly] to all State purchases. (9) Foreign firms in US markets are treated [equally] with their US counterparts. Distinguishing adverbs, adverbial phrases, and adverbial clauses The following sentences illustrate the difference between adverbs, adverbial phrases, and adverbial clauses. (10) I'll go to bed soon. (11) I'll go to bed in an hour. (12) I'll go to bed when I've finished my book. In the first example, "soon" is an adverb (as distinct from a noun or a verb), which is a type of adverbial. In the second sentence, the modifier "in an hour" has the same syntactic function (that is, to act adverbially and modify the base of the sentence "I'll go to bed"), though it does not contain an adverb. This modifier consists of a preposition and a determiner phrase, and functions as an adverbial, thus making it an adverbial phrase. In the third example, we see a whole clause functioning as an adverbial; it is termed an adverbial clause. Distribution Functionally, the term adverbial refers to all structures that can take the position of an adverb on a phrase structure level. Adverbs modify the functional categories that occur in a sentence and may also be treated as predicates which are functionally open and require one or more arguments to be satisfied. It has been argued that the distribution of adverbs is largely conditioned by their lexical nature or thematic properties. Classes of adverbials Main classes of adverbials are used to distinguish the functional properties of the adverbs within the phrase. Each class has subcategories, that refer more specifically to the syntactic and semantic properties of the adverbial. There is no distinct terminology for these classes used universally in literature, though adverbials are often classified into their