Relative clauses are a kind of clause that modifies nouns. Some examples are shown in (1).

Relative clauses are always adjuncts to a noun phrase. In (1), the nouns that are modified by the relative clauses are bolded. Any noun phrase can be modified by a relative clause.

Relative clauses also always have a gap. In (1), the gaps are marked with underscores. The gap can be in subject position, as in (1a) and (1b), in object position, as in (1c), or in adjunct position, as in (1d), but the gap will always have the same meaning as the noun that the relative clause modifies. We can illustrate this by rephrasing the complex sentences in (1) as two simple sentences, as shown in (2).

Relative clauses may use complementizer that, as in (1c), a question word such as who in (1a) or where in (1d), or neither, as in (1b). If it has neither a complementizer or a question word, it is called a reduced relative clause. Interestingly, it is not possible to use both that and a question word. This is shown in (3). The relative clause can be introduced by who as in (3a) or that as in (3b), but cannot be introduced by both at the same time, in either order, as shown in (3c)-(3d).

Relative clauses have very similar structure as embedded content questions. In fact, relative clauses that use question words look identical to embedded content questions—the only difference is their position in the main clause. Relative clauses modify nouns, but embedded content questions are in the argument position of a verb.

Factive clauses

Not all clauses that modify noun phrases are relative clauses. Another kind of clause, called a factive clause, also modifies noun phrases. Unlike relative clauses, factive clauses do not have a gap. Some examples of factive clauses are shown in (6).

Factive clauses have the same structure as embedded declarative clauses. The only difference is that they modify a noun, instead of being in the argument position of a verb.

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