If you read that and you’re shouting “THAT!” at the screen (or your phone, or your tablet, or whatever device you might be reading this on), this is probably for you.
If you read that and you have no reaction whatsoever, you might not care. Read if you want. I won’t know if you go no farther (further?) than this.
So. The “which/that” issue is one that (or which) editors struggle with (or not, depending on their background), but that other folks don’t think much about. Brits, for example. They don’t differentiate, as a rule, and use the two words interchangeably.
However, some of us (::raises her hand high::) had it drilled into us that “which” is “nonrestrictive” and “that” is “restrictive” and we have, for decades, some of us, dutifully gone on “which hunts” every time we start a new project, ensuring that all usages adhere to the tradition we learned in high school.
The higher the register of the work, the more likely it will be expected to adhere to the differentiation. Generally speaking, of course.
Here’s how it works.
Bring me the red cloak that is behind the door.
Presumably, there are other red cloaks elsewhere. I want the one that’s behind the door. I’m restricting the options. “Behind the door” is important, because it’s telling you which (HAHAHAHA SEE WHAT HAPPENED THERE) red cloak I want. Not the one on the hook. Not the one on the bed. I want the one that is behind the door.
Bring me the red cloak, which is behind the door.
Two things are at work here. First, there’s that comma after “cloak.” Second, there’s “which” instead of “that.” There’s only one cloak, we can assume, in this scenario. It happens to be behind the door. I’m telling you that as an extra bit of information; it is not restricting you to only one cloak, because, well, there IS only one cloak, and it happens to be behind the door. The comma is a clue that we’re about to get more information that’s additional, not required. “Which” is the word of choice in this situation.
The thing is—and I’m looking over my shoulder for the Ghost of Mrs. Capps (because if anyone would haunt me over this, it’d be her)—that distinction is often overlooked, especially once you get away from the formal register. I’ve gotten to the point where I make my editorial decision based on readability. If I have to reread the sentence because the guideline wasn’t followed, I change it and I comment to the client, explaining why I did. Or, I’ll query without changing.
Bring me the red cloak which is behind the door. [Do you mean there is only one, and it’s behind the door? Or are there more, and you want that specific one? If you mean the first, we need a comma after “cloak.” If you mean the second, I suggest changing “which” to “that” here.]
Lynne Murphy (@lynneguist on Twitter) wrote a highly informative post about this at her blog, “Separated by a Common Language.” Check it out if you’re so inclined.