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Commas with which, that and who

The Quick Answer
If the information provided by who or which clause is just additional information, then separate it from the rest of the sentence with commas. In other words, if you would happily put brackets around it, you must at least put commas around it.

When to Use Commas with Who and Which

Often there is confusion about when to use commas with who and which. The rule is: If the information provided by the who and which clause (called a relative clause) is just additional information (i.e., it is not required to identify the word it's linked to), then it must be separated from the rest of the sentence with commas.

In other words, if you'd happily put brackets around it, then you must at least put commas around it.

Examples:
  • His youngest daughter, who was born on 16 June 1972, swam the channel.
  • (The clause who was born on 16 June 1972 is just additional information about His youngest daughter – commas required.)
  • James Baker's cat made its own way home after it was accidentally left on the beach at Scarborough. James, who has lived in our village for 10 years, has just won the lottery.
  • (The clause who has lived in our village for 10 years is just additional information about James – commas required.)


    This is correct.
    The clause who has splashed out 850,000 on a villa near the town in the South of France is just additional information – commas required.
    (magazine article)

  • William Scott is a millionaire. William who bought his first house in the '80s is estimated to be worth more than 10 million pounds.
  • (The clause who bought his first house in the '80s is just additional information. It should be offset with commas).
Top Tip

Commas Are Being Used as Parentheses

If a clause just adds additional information, then it should be separated from the rest of the sentence with commas. You could equally use brackets or dashes.
(This is covered in the lesson Parenthesis.)
  • Manx cats, which live on the Isle of Man, have a longer life expectancy than normal domestic cats.
  • Manx cats (which live on the Isle of Man) have a longer life expectancy than normal domestic cats.
  • Manx cats – which live on the Isle of Man – have a longer life expectancy than normal domestic cats.

Try the Removal Test

If a clause just adds additional information, then it can be removed without any loss of meaning to the main sentence.
  • Manx cats have a longer life expectancy than normal domestic cats.
Beware

Beware of Ambiguity

Look at the sentences below. Both are grammatically correct, but they have different meanings.
  • Manx cats, which live on the Isle of Man, have a longer life expectancy than normal domestic cats.
  • Manx cats which live on the Isle of Man have a longer life expectancy than normal domestic cats.
The first example means that all Manx cats have a longer life expectancy than normal cats. (The clause is simply additional information telling the reader where Manx cats live.)
The second example means that only Manx cats living on the Isle of Man have a longer life expectancy (i.e., Manx cats that live elsewhere do not).