The grammatical function of the relative clause is to “modify” the noun phrase that contains it. In other
words, it describes or limits the meaning of the noun phrase.
Restrictive Relative Clauses
For example, in the sentence
A camera which has a solid-state image sensor is a digital camera.
the relative clause limits the noun phrase a camera to the specific kind of camera described by the information in the relative clause. Without that information, the sentence has an entirely different meaning. Compare the sentence below, without a modifying relative clause, with the one above:
A camera is a digital camera.
This sentence implies that all cameras are digital cameras, which is not true. The relative clause in the first sentence describes a very specific kind of camera. Its content is crucial to the meaning of the sentence. In grammatical terms, this kind of relative clause is known as a “restrictive” relative clause because the relative clause “restricts” or limits the meaning of a noun phrase in some way.
Nonrestrictive Relative Clauses
A second kind of relative clause is known as a “nonrestrictive” (or “appositive”) relative clause. Instead of restricting the meaning of a noun phrase, the nonrestrictive relative clause simply provides extra information about a noun phrase. In the sentence
Adobe Photoshop, which is a photo-editing software program, can be purchased at the bookstore.
the nonrestrictive relative clause explains what Adobe Photoshop is. It does not limit the meaning of Adobe Photoshop in any way. Without the relative clause the sentence is still true: Adobe Photoshop can be purchased at the bookstore.
Commas with Nonrestrictive Relative Clauses
In addition to the important meaning differences associated with restrictive and nonrestrictive relative clauses, notice that it is only the nonrestrictive relative clause that is surrounded by commas in written English. In the following sentence, the restrictive relative clause who use digital cameras is contained inside the nonrestrictive relative clause, which now hire adjusters.... The nonrestrictive relative clause provides elaboration on the noun phrase insurance companies, and the restrictive relative clause limits the noun phrase adjusters to only those who use digital cameras.
Insurance companies, which now hire adjusters who use digital cameras, can complete estimates much faster than in the past.
Relative Clauses in Discourse
In addition to their roles inside sentences, relative clauses play an important role in “discourse.” Discourse refers to a sequence of sentences that make up a message, a story, or a text. Sentences with relative clauses help to keep information flowing. They introduce a new topic or set the stage for the development of a topic. They help to compare different kinds of items. They provide elaboration or background information about a noun phrase. And they can refer back to earlier information in the discourse.