Which to Use—Which or That?

These two words are often used incorrectly, and it’s not too hard to understand when you want that instead of which. The word that is used with what’s called a restrictive clause—meaning the phrase that’s following this word is necessary to the meaning of the sentence. For example:

• All the books that are about dogs are on that shelf. [This means every book about dogs is on the shelf.]
• I went to see the movie that had a lot of singing. [This means you specifically chose a movie with singing as opposed to another movie without singing.]
• He liked my novel that reminded him of home. [This means this novel reminded him of home as opposed to others that didn’t.]

Now, look at the difference in meaning by using which instead (and notice the offsetting commas). Which is used with a nonrestrictive clause, which means the phrase it’s a part of isn’t essential to the meaning but just adds something already known to the first phrase (I just gave you an example in this sentence).

• All the books, which are about dogs, are on that shelf. [This means all the books are on the shelf, and they just happen to be about dogs. The emphasis here is on the fact that the books are on the shelf. It doesn’t really matter what they’re about. The fact that they’re about dogs is an additional bit of information that doesn’t change the essential meaning of the sentence.]
• I went to see the movie, which had a lot of singing. [This means the singing isn’t singling out which movie you saw. The movie you saw just happened to have singing, but it doesn’t change the essential fact of the sentence—that you went to the movies.]
• He liked my novel, which reminded him of home. [This means he liked my novel not because it reminded him of home—he just liked it. And it also reminded him of home.]

In most instances, you are going to have a comma before which. That’s because you are introducing a nonrestrictive bit of info. Some grammar gurus feel it’s fine to interchange which and that in many circumstances, but if you follow this generally accepted, rule, you should do fine for the most part, which is a good goal to have (the which put here to belabor my point!).