Modification is a very general syntactic function that is relevant at every level of structure. Any syntactic element that is “optional” and which is not a Complement is probably functioning as a Modifier. Since Modification is a function, it describes what a linguistic element does, and is therefore defined independently of what that element is. Thus, the function of Modification can theoretically be filled by elements of any syntactic category. In practice, there are certain categories that seem particularly prone to filling Modification functions, though all of these can fill other functions as well. For example, when we think of Modification of nouns, we immediately think of adjectives. However, as we've seen in Chapter 9, adjectives may serve as Complements, as Heads of NPs, and other functions as well. Furthermore, any number of syntactic categories other than adjectives can also function to Modify nouns in NPs. These include other nouns, prepositional phrases, and whole clauses, as we will see below.

Modification serves an important role in communication. The thoughts that people need to communicate with one another seldom match the idealized conceptual scenes evoked by individual lexemes exactly. Rather, they are rich, nuanced, often unique representations involving detail that may not be a part of the sparse images evoked by particular nouns and verbs.