(2) If you'd be happy to put brackets around it or delete it, then use a comma.
(3) If you can't put it in brackets or delete it, then don't use a comma.
- The car which hit the snowdrift is a write-off. (which hit the snowdrift – restrictive clause, i.e., required to identify the car – not offset with commas)
- The car that hit the snowdrift is a write-off. (restrictive clause – which can be replaced with that)
When Not to Use Commas with Who and WhichThere is often confusion about when to use commas with who and which. Unfortunately, there is no simple rule. Sometimes there should be commas and sometimes there should not. The rule is: When the information provided by the clause is required to identify the person (or thing), then there are no commas.
The examples will make this clearer.
- The boy who broke our window brought some flowers to the door. (The clause who broke our window is required to identify the boy – no commas.)
- The driver who stole indicator bulbs for his own car was given a formal warning. (The clause who stole indicator bulbs for his own car is required to identify the driver – no commas.)
- Mr Jeremy Buxton of 16 High Street who was born on the Isle of Wight is the second person from the village to represent England at Cluedo. (The clause who was born on the Isle of Wight is not required to identify Mr Jeremy Buxton of 16 High Street – there should be commas around this clause.)
- Sarah has always been close to her parents who live in the same village as us. (The clause who live in the same village as us is not required to identify Sarah's parents – comma required before who.)
should be ...parents, who live... (unless she has other parents)
(letter in a newspaper)
- Experience is a comb which nature gives us when we are bald.
- The man who doesn't read good books has no advantage over the man who can't read them. (Mark Twain)
- It is only shallow people who do not judge by appearances. (Oscar Wilde)
- The man who can dominate a London dinner-table can dominate the world. (Oscar Wilde)
- One should never trust a woman who tells one her real age. A woman who would tell one that would tell one anything. (Oscar Wilde)
That with a Restrictive ClauseWhen a clause is necessary for identification, it is called a restrictive clause. It is restrictive because you have to use it. There are never commas around a restrictive clause. When introducing a restrictive clause, the words who and which can be replaced with that. (Therefore, there are never commas around a clause which starts with that.)
- The boy who broke our window bought me some flowers.
- The boy that broke our window bought me some flowers.
- The PC which keeps breaking down is under guarantee.
- The PC that keeps breaking down is under guarantee.
This infographic might make it easier to understand:
Remove It AltogetherQuite often with a restrictive clause, you can remove the who, which or that altogether.
- The reprimand which you received was justified.
- The reprimand that you received was justified.
- The reprimand you received was justified.
Beware of AmbiguityLook at the sentences below. Both are grammatically correct, but they are slightly different in meaning.
- Sarah has always been close to her sister who lives in the same village.
- Sarah has always been close to her sister, who lives in the same village.
In the first example, you can assume that Sarah has more than one sister and that Sarah is close to the one that lives in the same village. In the second example, you can assume that Sarah has only one sister. As it happens, she lives in the same village as Sarah.