That or Which?

I have always struggled with deciding whether to use that or which. In Woe Is I, O'Connor says that "Some people consider which more refined or elegant. Not so!" (3). I am definitely guilty of that one. I don't know why, but which just seems better than that. Maybe it's because it has more letters or because it sounds more proper. Nonetheless, I always seem to use which over that.

However, there is a place for both of them. As O'Connor points out, "In fact, that is more likely to be grammatically correct than which" (3). Who knew?

So what is their correct usage? That goes before a clause that is necessary for sentence to make sense, and, thus, it does not need a comma before it. A which clause, on the other hand, does go in commas, because it can be removed from the sentence without impairing the meaning. And by the way, a clause just means that it is "a group of words with its own subject and verb" (O'Connor 3).

The best way to know if a clause is necessary or not is to take it out. If you remove it and the sentence still makes sense, use which at the beginning of it. But, if you remove the clause and the whole point of the sentence is lost then use that.

In order to remember this tricky part of grammar, O'Connor gives a helpful little poem:
"Commas, which cut out the fat,
Go with which, never with that" (4).

Hopefully, now, I'll never get mixed up by which or that again!