What are the rules regarding the use of a comma before the word which? I thought both parts of the sentence had to stand alone for it to be used but am working with an editor who has put it in where this is not the case. I just want to have a better understanding of why.
When which is used as a relative pronoun in an undefining relative clause, a comma is usually used.
An undenfining clause adds extra information, but the sentence would still make sense without it.
He bought a Bugatti Veyron sports car, which cost £839,285!
The second clause in such a sentence is subordinate and therefore cannot stand on its own.
If this doesn't help, can you quote the sentence you're concerned about?
I am not a teacher.