A relative clause is a type of subordinate clause which is attached to a head noun within a noun phrase. There are two types of relative clauses – restrictive (or defining) and non-restrictive (or non-defining). A restrictive relative clause is required to identify what is being referred to.

Study the following sentence.

• The museum which we visited yesterday was very large.

Here the relative clause is which we visited yesterday, and it is attached to the head noun museum. This relative clause is restrictive because it is required for identification; without it, the sentence would read The museum was very large which does not identify the museum under discussion.

More examples are given below

The woman who does my hair has moved to another hairdresser’s. (Here the relative clause who does my hair is restrictive because it is required for the identification of the woman.)

In contrast, a non-restrictive or non-defining relative clause is not required for identification; it serves only to provide additional information. Consider the sentence given below:

• James, who is a chain smoker, has been diagnosed with throat cancer.

Here the relative clause who is a chain smoker is not required for the identification of James who has already been identified by his name.

Identifying relative clauses usually follow immediately after the nouns that they modify. They are not separated by commas in writing. Non-identifying clauses are separated by commas.