If the information provided by the clause introduced by who or which is necessary to identify 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. When such a clause (called a relative clause) is necessary for identification purposes, it is called a restrictive clause, and the who or the which can be replaced with that. For example:|
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.
should be ...parents, who live... (unless she has other parents)
(letter in a newspaper)
THAT WITH A RESTRICTIVE CLAUSE
When 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.)
This infographic might make it easier to understand:
REMOVE IT ALTOGETHER
Quite often with a restrictive clause, you can remove the who, which or that altogether.
BEWARE OF AMBIGUITY
Look at the sentences below. Both are grammatically correct, but they are slightly different in meaning.
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.