Q: I've been writing for a long time and always assumed which and that were interchangeable, but I've recently been told that isn't the case. How do I make sure I'm using the right word? —Anonymous
The battle over whether to use which or that is one many people struggle to get right. It's a popular grammar question and most folks want a quick rule of thumb so they can get it right.
Here it is:
If the sentence doesn't need the clause that the word in question is connecting, use which. If it does, use that. (Pretty easy to remember, isn't it?) Let me explain with a couple of examples.
Our office, which has two lunchrooms, is located in Cincinnati.
Our office that has two lunchrooms is located in Cincinnati.
These sentences are not the same. The first sentence tells us that you have just one office, and it's located in Cincinnati. The clause which has two lunchrooms gives us additional information, but it doesn't change the meaning of the sentence. Remove the clause and the location of our one office would still be clear: Our office is located in Cincinnati.
The second sentence suggests that we have multiple offices, but the office with two lunchrooms is located in Cincinnati. The phrase that has two lunchrooms is known as a restrictive clause because another part of the sentence (our office) depends on it. You can't remove that clause without changing the meaning of the sentence.
Let’s look at another example:
The time machine, which looked like a telephone booth, concerned Bill and Ted.
The time machine that looked like a telephone booth concerned Bill and Ted.
In the first sentence (thanks to the use of which), the time machine concerned Bill and Ted. It also happened to look like a telephone booth. In the second sentence (which uses the restrictive clause), Bill and Ted are concerned with the time machine that looks like a telephone booth. They aren't concerned with the one that looks like a garden shed or the one that looks like a DeLorean (Marty McFly may have reservations about that one).
Now that you've learned the rule, let's put it to a test:
1. The iPad (which/that) connects to the iCloud was created by Apple.
2. The issue of Writer's Digest (which/that) has Brian A. Klems’ picture on the cover is my favorite.
The correct answers are:
1. The iPad, which connects to the iCloud, was created by Apple. (All iPads connect to the iCloud, so it's unnecessary information.)
2. The issue of Writer's Digest that has Brian A. Klems picture on the cover is my favorite. (Your favorite issue of Writer's Digest isn't just any issue, it's the one with me on the cover.)
OK, so I've never been on the cover of Writer's Digest, but that doesn't change the fact that it's necessary for you to understand the context of your clauses, a key covered in most grammar books. If the information is essential, use that. If it's just additional information that's useful but unnecessary, use which.