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 clause is just additional information - i.e.,
not required to identify the person (or thing) - then it is separated from the rest of the sentence with commas.
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).
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WHY COMMAS?
If a clause just adds additional information then it should be separated from the rest of the sentence with commas. However, 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.
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.
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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).
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