Relative clauses give us information about the person or thing mentioned. Defining relative clauses give us essential information – information that tells us who or what we are talking about. ... We usually use a relative pronoun or adverb to start a defining relative clause: who, which, that, when, where or whose.

What is a defining relative clause?

A defining relative clause usually comes immediately after the noun it describes. We usually use a relative pronoun (e.g. who, that, which, whose and whom) to introduce a defining relative clause (In the examples, the relative clause is in bold, and the person or thing being referred to is underlined.):

What is the purpose of a non defining relative clause?

Non-defining relative clauses give us extra information about someone or something. It isn't essential for understanding who or what we are talking about. My grandfather, who's 87, goes swimming every day. The house, which was built in 1883, has just been opened to the public.