Defining and non-defining relative clauses

Look at these two sentences. How do the commas change the meaning?

1 My sister who lives in Melbourne has two children.
2 My sister, who lives in Melbourne, has two children.

How many sisters have I got in each case? In sentence 1, I have more than one sister. In sentence 2, I have only one sister.

A relative clause gives extra information about a noun in the main clause. A relative clause is defining or non-defining.

The information given in a defining relative clause is essential to the meaning of the sentence: it makes it clear which person (or thing) we are talking about:
My sister who lives in Melbourne has two children.

The information given in a non-defining relative clause is not essential to the meaning of the sentence. This extra information is separated by commas in writing, and intonation in speaking:
My sister, who lives in Melbourne, has two children.


A relative clause is introduced by a relative pronoun: who (whom), which, that, whose, or there may be no relative pronoun. The choice of relative pronoun depends on whether:
(1) it is the subject or object or possessive of a relative clause
(2) it refers to a person or thing
(3) the relative clause is defining or non-defining

Relative pronouns in defining relative clauses


Relative pronouns in non-defining relative clauses


Notes:
The pronouns in brackets are less commonly used.
Whom is formal and is used mainly in writing.
That can never be used in non-defining relative clauses.