A relative clause is one that’s connected to the main clause of the sentence by a word such as who, whom, which, that, or whose. For example:
It reminded him of the house that he used to live in.
The items, which are believed to be family heirlooms, included a grandfather clock worth around $3,000.
There are two types of relative clause: restrictive (or defining) relative clauses and nonrestrictive (or nondefining) relative clauses. The difference between them is as follows:
- A restrictive relative clause provides essential information about the noun to which it refers. It cannot be left out of the sentence without affecting the meaning. The highlighted section of the first sentence above is a restrictive relative clause. If it were left out, the sentence would not make sense:
It reminded him of the house. [which house?]
- A nonrestrictive relative clause provides information that can be left out without affecting the meaning or structure of the sentence. The highlighted section of the second sentence above is a nonrestrictive relative clause: if it were left out, the sentence would still make perfect sense:
The items included a grandfather clock worth around $3,000.
You should not put a comma before restrictive relative clauses. On the other hand, nonrestrictive relative clauses should be separated from the rest of the sentence by a comma or commas. For example:
A list of contents would have made it easier to steer through the book, which also lacks a map.
Bill, who had fallen asleep on the couch, suddenly roused himself.
Restrictive relative clauses can be introduced by that or which when they are referring to things rather than people, but in American English the predominant choice is to use that:
The coat that Dan had on yesterday was new.
In this sentence, the writer is identifying the coat by saying it’s the one Dan was wearing yesterday, as opposed to any other coats he might own.
Nonrestrictive relative clauses must always be introduced by which and never by that:
The coat, which Dan had on yesterday, was made of pure alpaca and cost a fortune.
In this sentence, there’s no need to identify the coat—it’s already been mentioned. But the writer is providing a bit of background context by telling us that Dan was wearing it yesterday.
Read more about clauses.