We can use relative clauses to join two English sentences, or to give more information about something.

I bought a new car. It is very fast.
→ I bought a new car that is very fast.

She lives in New York. She likes living in New York.
→ She lives in New York, which she likes.

A defining relative clause tells which noun we are talking about:

• I like the woman who lives next door.
  (If I don't say 'who lives next door', then we don't know which woman I mean).

A non-defining relative clause gives us extra information about something. We don't need this information to understand the sentence.

• I live in London, which has some fantastic parks.
  (Everybody knows where London is, so 'which has some fantastic parks' is extra information).

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