A clause which clearly defines the antecedent is known as a defining relative clause. (The antecedent is the noun that the relative pronoun refers to.) On the other hand, a clause which merely gives additional information about the antecedent is known as a non-defining relative clause.

Examples of defining relative clauses are:

• Charles Dickens is a writer whom the whole world admires.
• The book which you gave me is very interesting.
• The diamond necklace which I bought for my wife was very expensive.

Examples of non-defining relative clauses are:

• My brother, who lives in Canada, is an accomplished painter.
• Raj, who is my friend, is a handsome guy.

Notes

The relative pronoun introducing a defining relative clause can be omitted in an informal style.

• The diamond necklace I bought for my wife was very expensive.

The relative pronoun introducing a non-defining relative clause cannot be omitted.

• Maria, who babysits my kids, is my aunt’s daughter. (NOT Maria, babysits my kids, is my aunt’s daughter.)

The relative pronoun cannot be omitted in a non-defining relative clause.

If the non-defining relative clause occurs in the middle of the sentence, it must be preceded and followed by a comma.

• My son, who makes models from clay, has won many prices.

This use of the comma often distinguishes between defining and non-defining relative clauses.