Relative clause

A relative clause is a dependent clause introduced by a relative pronoun — who, whom, whose, which or that — that modifies a noun, either to identify which one is meant or to add detail about one already identified.

There are two kinds, and confusing them is one of the few punctuation errors that changes the facts.

Restrictive — no commas. The clause identifies which one you mean, so the sentence loses its sense without it.

The drafts that she had annotated went back in the folder.

Only the annotated ones went back. The clause is a bound modifier: it cannot be lifted out.

Non-restrictive — commas. The noun is already identified, and the clause adds something extra.

The drafts, which she had annotated, went back in the folder.

All of them went back, and by the way they were annotated. The clause is a free modifier, removable without damage.

The usual guidance in American usage is that for restrictive, which for non-restrictive; British usage allows which in both, but the commas still do the work. The commas are the real signal. If you can drop the clause and the sentence still means what you intended, use them; if you cannot, do not.

A non-restrictive relative clause and an appositive phrase do almost the same job, and the appositive is usually shorter: the drafts, all of them annotated, rather than the drafts, which she had annotated.