Use which for things and who for people. Use that for things and, informally, for people.|
When do you need a comma before which and who?
If the who or which clause is just additional information (i.e., you would be happy to put it in brackets), then you should offset it with commas.
Which, That and WhoThe words which, who, and that are grammar villains — they are often the cause of grammar errors. Most commonly, this stems from confusion over whether to use a comma before which or who. Unfortunately, the rules are not simple. They are explained in more detail in the following lessons:
Which, who and that.
(A lesson providing an overview of which, that, and who)
Commas before which and who.
(A lesson focusing on when to use commas with which and who)
No commas before which and who.
(A lesson focusing on when not to use commas with which and who)
These lessons are all quite similar, but they approach the issue from slightly different perspectives.
Select the correct version:
THAT = WHICH (WITHOUT A COMMA)
Here is a quick summary of when to use a comma with which (and who):
Comma After. If the clause (shown in bold below) is required to identify whatever it follows (car in this example), then there are no commas.
The car which I drove on Tuesday has been sold.
The car, which I drove on Tuesday, has been sold.
Use That If You Want. If you think the word that sounds better than which, then use that (provided there are no commas).
The car that I drove on Tuesday has been sold.
No Comma After. If the clause is just additional information, because whatever it follows has already been identified (John's red Mustang in this example), then use commas.
John's red Mustang, which I drove on Tuesday, has been sold.
GRAMMAR CHECKERS WON'T HELP ON THIS ONE
Grammar checkers do not understand what has been written. They perform a mathematical grammar check on writing. This is why they are rubbish at determining whether there should be a comma before which or who. Many checkers encourage you to use that, and, if you don't, they offer you the with-comma version. This relies on your knowledge or instinct to get it right. It may also cramp your style as it does not offer the non-comma version.
A GREAT TIP
If you're happy with putting brackets around the clause that starts who or which, then you should be using commas. If brackets would confuse things, then don't use commas. This tip works because both commas and brackets can be used to separate additional information from the main elements of sentence.