What is a relative clause?

Defining and non-defining relative clauses refer to preceding nouns and are introduced by the pronouns who, whom, whose, which or that. Relative clauses usually come after the nouns that they describe:

The woman who answered the door was about forty years old.
Madrid, which is the capital of Spain, lies on the river Manzanares.

Sentential relative clauses do not refer to preceding nouns but comment on what has been said before:

I didn't take any waterproof clothing with me, which was a mistake.

In relative clauses, we do not use personal pronouns to refer to the nouns described:

The woman who she answered the door was about forty years old.
Madrid, which it is the capital of Spain, lies on the river Manzanares.