Comma before "Which" and "Who"

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The Quick Answer
When do you use a comma before "which"?

An Extremely Simple Check

If you can replace your "which" with "that," don't use a comma before your "which."

A Simple Check

Here is a simple process to help your decision on whether to use a comma before "which."
  • First: Look at the clause starting with "which."
  • Check 1: If you'd be happy to put parentheses (brackets) around it or even delete it, then offset it with commas.
  • Check 2: If you can't put it in brackets or delete it, then don't use commas.
(These checks apply equally to "who.")

A Bit More on Using a Comma before "Which"

Here's a slightly longer explanation.

If the information provided by the clause introduced by "who" or "which" is necessary to define the person or thing it is describing (i.e., it's not just some extra information you could easily remove), then the clause is not offset with commas.
  • The car which hit the snowdrift is a write-off.
    • (The clause "which hit the snowdrift" is required to define the car, so it is not offset with commas.)
When a clause starting with "which" (called a "relative clause") is necessary for identification purposes, it is called a restrictive clause or a defining clause. With a restrictive clause, the "which" can be replaced with "that." For example:
  • The car that hit the snowdrift is a write-off.
    • (With a restrictive clause, "which" can be replaced with "that." In the US, the practice of starting a restrictive clause with "which" is highly unpopular, to the extent that many consider it a mistake.)
comma before which
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When to Use Commas before "Which" and "Who"

There is often confusion about when to use commas with "which" and "who." Unfortunately, there is no simple rule. Sometimes, there should be commas, and sometimes there should not. Here is the rule:
The Rule for Using Commas with "Which" and "Who"

When the information provided by the clause starting with "which" or "who" is required to define the person (or thing), then there are no commas. If the clause provides just additional information, then use commas.

Examples of "Which" and "Who" Clauses without Commas

Here are some examples of "who" clauses:
  • The boy who broke our window brought some flowers to the door.
    • (There are no commas because the clause "who broke our window" is required to define "the boy.")
  • The driver who stole indicator bulbs for his own car was given a formal warning.
    • (There are no commas because the clause "who stole indicator bulbs for his own car" is required to define "the driver.")
  • Jeremy Buxton of 16 High Street who was born in Boston is the second person from the village to represent the country at basketball.
    • (The clause "who was born in Boston" is not required to define "Jeremy Buxton of 16 High Street." Therefore, 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 define Sarah's parents. Therefore, a comma is required before "who.")
The last example above is a real example from a newspaper. Let's examine it in more detail.

comma before which example 1

The clause "who live in the same village as us" is just additional information about Sarah's parents. Therefore, there should be a comma before "who." However, if Sarah had adoptive parents too, then this example would be correct (i.e., correct without a comma). The clause would then be defining which set of parents she has always been close to. In other words, Sarah would have two sets of parents:
  • her parents who live in the same village
  • her parents who live somewhere else
The key point here is that using or not using a comma before "which" or "who" is not usually a grammar mistake, but it does change the meaning of your sentence.

Real-life Examples

Here are some real-life examples of clauses that are required to define. (These are called restrictive clauses or defining clauses.)
  • The man who doesn't read good books has no advantage over the man who can't read them. (Author Mark Twain)
  • It is only shallow people who do not judge by appearances. (Playwright Oscar Wilde)
  • The man who can dominate a London dinner-table can dominate the world. (Playwright 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. (Playwright Oscar Wilde)

Examples of "Which" and "Who" Clauses with Commas

Remember that if the information provided by the "who" and "which" clause is just additional information (i.e., it is not required to define the word it is modifying), then it must be separated from the rest of the sentence with commas.

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

Examples:
  • His youngest daughter, who was born on 16 June 1972, was selected for astronaut training.
    • (The clause "who was born on 16 June 1972" is just additional information about "His youngest daughter." Therefore, commas are 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." Therefore, commas are required. We know who James is because he is defined in the first sentence.)
  • 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).

"Which" and "That"

You will notice that all of the examples in the section above use "who" and not "which." We have used "who" because using "which" without a comma is an unpopular practice (especially in the US). Here is an example of "which" without a comma:
  • Experience is a comb which nature gives us when we are bald.
    • (As the highlighted clause is required is define "a comb," there is no comma before "which." Brits are fine with this. Americans are not. They would prefer "that nature gives us when we are bald.")

"That" with a Restrictive Clause

When a clause is necessary for defining (or identification), it is called a restrictive clause. It is described as "restrictive" because it restricts the meaning of the word it modifies. It is also called a "defining clause," which, I think, is a more accurate term. There are never commas around a restrictive clause. When introducing a restrictive clause, the words "who" and "which" can be replaced with "that." (NB: There are never commas around a clause which starts with "that.")

Here is an example with "who":
  • The boy who broke our window bought me some flowers.
  • The boy that broke our window bought me some flowers.
    • (Using "that" for people is best avoided in formal writing.)
Here is an example with "which":
  • The PC which keeps breaking down is under guarantee.
    • (This is unpopular, especially in the US.)
  • The PC that keeps breaking down is under guarantee.
Quite 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.

Commas Are Being Used as Parenthetical Punctuation

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. These are all types of parenthetical punctuation.
  • 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.
Remember that 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.
Read more about your choice for parenthetical punctuation.

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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See Also

Using which, that and who Commas with which, that and who