A simple rule and some exceptions.
Here's a Quick and Dirty Tip for the simple rule: If you think of the Wicked Witch (Which) of the West from The Wizard of Oz, you know it’s OK to throw her out. She's bad, so we want to get rid of her. We're going to throw out the wicked witch, just like you can throw out the "which" part of your sentence. You won’t change the meaning of the sentence without the "which" phrase. So, you can throw out the which (or witch) phrase, commas and all. Think of lifting it up by the commas and taking it away. If you can do that, and it doesn't change the main meaning of the sentence, then you know that "which" is the right choice. If you try to throw out the phrase and it does change the meaning of the sentence, then you know the right choice is "that" instead of "which" because it's a restrictive element.
That's the simplified rule that I find works for a lot of people who get frustrated trying to decide which word to use.
“Which” Versus “That”— Advanced
You should also know that the situation is more complicated than what I just explained. That was the safe rule. You'll never go wrong with it, but some authorities say that “which” can actually be used for both restrictive and non-restrictive phrases, and it's actually very common for people who use British English to use "which" when American speakers would use "that." The distinction between the two just hasn't held up in British English the way it has in American English.
So if you're British, know that Americans might think you've made a mistake when you use "which" with a restrictive element, or they may be dazzled by your accent and not even notice. We're like that.
And if you're American but you love the sound of the word "which" in all your sentences and want to use it, if anyone challenges you, you can just say that you're using British English. But I do think the distinction bewteen the two words is useful because as I explained, they convey different ideas.
Consider these two examples:
Diamonds that are expensive make a great gift.
Diamonds, which are expensive, make a great gift.
In the first example—Diamonds that are expensive make a great gift—I'm saying there are two kinds of diamonds: some that are expensive and some that aren't expensive.
In the second example—Diamonds, which are expensive, make a great gift—I'm saying that to me, all diamonds are expensive.
And I think that's a useful distinction.
“That” and “Which”—Relative Pronouns
I'll finish with a little grammatical aside. When they're used in the way we've been talking about, “that” and “which” are called relative pronouns. I know it seems weird because you usually think of pronouns as words such as “he” and “she.” Relative pronouns don't get talked about as much as other pronouns, but they are real pronouns. They head up subordinate clauses. Other relative pronouns include “who,” “whom,” “why,” “where,” and “when.”
To sum up, the simplest rule is to choose the relative pronoun “that” when you can't get rid of the element and the relative pronoun “which” when you can get rid of the element and it won't change the main meaning of the sentence. Remember that it's always safe to throw out the “whiches.”
The Grammar Devotional
Also, I'm delighted to tell you that my second book, The Grammar Devotional, is in stores. It's not available as an audiobook because it has puzzles and quizzes, so you have to get the print or e-book edition. It has 365 easy tips, quizzes, cartoons, and puzzles to help you be a better writer.
Of course, “which” and “that” have other uses too. Here are some other podcasts we've done on these words:
Image courtesy of Shutterstock.