Most human languages have certain constructions which are “expandable” to any degree the speaker wishes to take them. We have already seen several different ways in which English NPs can be “expanded” without limit; two of these are illustrated in (1).

1. (1) a my favorite uncle's youngest daughter's oldest son's best friend's new bicycle

2. b the portrait at the top of the stairs in that castle on a hill beside the river

In order to account for the possessive NP construction in (1a), we proposed a recursive Phrase Structure rule (chapter 6, section 6.4.2). A recursive rule is one which permits a mother node of some category (in this case, NP) to have a daughter of the same category. Recursive structures can also be generated when there are two phrasal categories such that each of them can dominate the other. For example, as illustrated in (1b) an NP may optionally contain a PP modifier, which will normally contain an NP object, which in turn may contain a PP, and so on.

The existence of recursive structures is a very important aspect of human grammatical systems. A particularly interesting type of recursion arises when one clause (category S) is embedded inside another. This is a major focus of study for syntacticians. Over the past forty years a huge amount of research has been devoted to understanding the grammatical properties of such structures, and it would obviously be impossible to review all of this research in a single chapter, or even in a whole book.