To understand the difference between “which” and “that,” first you need to understand the difference between a restrictive element and a non-restrictive element, because the simple rule is to use “that” with a restrictive element and “which” with a non-restrictive element. OK, don't worry; it's easy.
Restrictive Clauses and Nonrestrictive Clauses
A restrictive element is just part of a sentence you can’t get rid of because it specifically restricts the noun. Here's an example:
Desserts that contain chocolate please Squiggly.
The words “that contain” restrict the kinds of desserts we're talking about. Without those words, the meaning of the sentence would change. Without them, we'd be saying that all desserts please Squiggly, not just the ones with chocolate.
Desserts that contain chocolate please Squiggly.
Restrictive elements aren't surrounded by commas.
Here's another example:
Jackhammers that don't have noise-dampening technology are on sale.
We can't get rid of the words “that don't have noise-dampening technology” because then we'd be saying all jackhammers are on sale, not just the special ones; so that means the phrase is restrictive.
And another example:
Dogs that howl make me crazy.
I can't get rid of the words “that howl” because then I'd be saying all dogs make me crazy, not just the ones that howl, which isn't true. I love dogs.
On the other hand, a non-restrictive element is something that can be left out without changing the meaning of the sentence. A nonrestrictive element is simply additional information.
Chocolate desserts, which are his favorites, please Squiggly.
Leaving out the words “which are his favorites” doesn’t change the meaning of the sentence. With or without the words “which are his favorites,” we know that chocolate desserts please Squiggly. Nonrestrictive elements are surrounded by commas.
Here's another example:
Jackhammers, which are useful for breaking up concrete, are on sale.
We could throw out the words “which are useful for breaking up concrete” and the meaning of the sentence wouldn't change. Those words are just extra, meaning they're non-restrictive, surrounded by commas, and “which” is the right word choice.
Here's another example:
Dogs, which make great companions, are usually furry.
Again, we could throw out the words “which make great companions” and not change the meaning of the sentence. Dogs are still usually furry. Those words, “which make great companions,” are just extra, meaning they're non-restrictive, surrounded by commas, and “which” is the right word choice.
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.”
Summary
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.
Related Episodes
Of course, “which” and “that” have other uses too. Here are some other podcasts we've done on these words:
Can You Start a Sentence with “Which”?
Additional Resources
Relative Pronouns from the Online Writing Lab at Purdue.
Image courtesy of Shutterstock.
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