Sometimes, you need a comma before which and who. The rule is this: If the information added by the who or which clause is just additional information (i.e., it's not essential to identify another word in the sentence), then you should offset the clause with commas.
Using Which, Who and ThatThis page is about the relative pronouns which, who, and that. They are used to link information (in the form of a phrase or a clause) to another word in the sentence.
Who is used for people. Which is used for things, and that can be used for either. It is quite unfashionable to use that for people. (The consensus seems to be that using that for people is still acceptable in speech and informal writing, but you should avoid doing it in formal writing.)
- The man who swam the channel. (The clause who swam the channel is linked to The man. As The man is a person, the clause starts with the relative pronoun who. That could also have been used, but it runs the risk of annoying readers.)
- The PC which keeps breaking down is under guarantee until March. (which keeps breaking down is a clause. It adds information about (i.e., links to) The PC.)
- The vicar which was on BBC1 last night used to be our local vicar. (A vicar is a person. Therefore, who should be used and not which.)
- In Kent, a man who shot a swan was jailed for 6 months.
- Please accept my resignation. I don't want to belong to any club that will accept me as a member. (Groucho Marx quote) (A club is a thing. Therefore, which will accept me as a member would have been okay. Remember, that can sometimes replace which.)
Commas with Which and WhoThe biggest issue for native English speakers is knowing when to use a comma before which and who. Unfortunately, the ruling is not simple.
When who or which introduces a restrictive phrase or clause (i.e., additional information that is required to identify the word it is linked to), then the who or the which can be replaced by that. If the who or which introduces information that is not essential to identify the word it is linked to (called the antecedent), then the who or the which will be offset with commas and it cannot be replaced with that.
This infographic might make it easier to understand:
Read more about using commas with who and which.
What Are Relative Pronouns?Which and who are relative pronouns. There are others, but these two are the most common.
Read more about the different types of pronouns.
The other relative pronoun covered in this section is that. All three (which, who, and that are used to link to another word in the sentence (very often the one directly to the left) to add information about that word.
Do Not Start a Sentence with Which or Who (Unless it's a Question)Do not start a sentence with words like which and who (unless it is a question).
- Living in Scotland is cheaper than living in England. Which is lucky because I live in Dumfries.
should be ...extra room, which gives...
Whose and Who'sWho's is a contraction of either who is or who has. It has no other uses.
- Who's coming to fix the bed? (who is)
- Who's eaten the last muffin? (who has)
- I met the inspector who's delivering tomorrow's briefing. (who is)
Whose, on the other hand, usually sits before a noun to introduce information or ask a question relating to ownership.
- Whose bike was expensive? (bike – noun)
- Carl knows the girl whose phone was stolen. (Whose in this example is a relative pronoun. It is introducing information relating to ownership of the phone.)
(Whose in this example is an interrogative pronoun. It is asking a question about the ownership of the bike.)
Read more about who's and whose.