The confusion between That and Which is probably the most common usage error of all, and you will find examples of it in many texts. Which of these two sentences is correct: “This is the band that made the album I bought,” or “This the band which made the album I bought”? While the error is easy to fix, the concept behind the usage may be a bit challenging. We will try to simplify it.

That is used to begin a restrictive phrase. What this means is that there is more than one of something in a particular category, and you want to tell the reader which one you are describing. For example, assume that there are multiple bands in the world. You use that to restrict, or limit, the category of “bands” down to one of the category. In a sense, you are telling the reader, or listener, “Of all the bands, I mean this particular band.” The question to ask yourself is: “Am I telling the reader which one?”

Which is used to provide further clarification about something, to describe it, or to re-name it. When using which, your reader or listener already knows which item in a category you are describing or there is only one item in the category. As such, which starts a non-restrictive phrase, meaning you are not restricting a category to indicate a particular item in the category. For example, “I love this album, which is from the band I was telling you about.” Here, the reader/listener already knows which album you are indicating. You don’t have to limit the category “albums” to tell the reader which particular album is being described. The question to ask yourself is: “Does the reader already know which one?”

What’s the big deal? The “big deal” is understanding. Consider these two correct sentences with different meanings: “Please steal the red car that is in the garage.” “Please steal the red car, which is in the garage.” In the first case using that, you have more than one red car. You want the thief to steal only the one in the garage. You are restricting the category called “red cars” to indicate a particular red car. In the second sentence, using which, you only have one red car, and you are providing the thief with some additional information about it, i.e., its location. If I am the thief, and you tell me the first sentence, I will respond, “Hey, I like red cars. Where are the others? I’ll take them all!” If you tell me the second sentence, I, being a thief who loves red cars, will respond, “Bummer. You only have one red car for me to steal. Thanks for telling me where it is.”

Quick note about grammar checkers: If you are using Word’s grammar checker (always a risky idea), you may be told that there is a problem with your use of which, and the solution is to put a comma before that word. Yes, you do need a comma before which” when using which to begin a descriptive phrase. However, the problem may not be the comma but using the wrong word. When we edit, we always go on a “which hunt” to make sure the right word is being used. You grammar checker might not catch this error, but we do.