The question of when to use which vs. that is one of the most common issues that people studying for the GMAT face. I’ve done some hunting around on the web, and while there are a lot of articles explaining the distinction, most of them present the issue is much more complicated terms than is necessary. Knowing the grammar behind the rule might occasionally come in handy, but the reality is that most of the time it’s pretty irrelevant. In this post, I’m going to give you the shortcut.

The most important thing to know is that which follows a comma and that does not. In other words, comma = which, no comma = that.

Incorrect: The treaty of Tordesillas, that was signed on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile.

Correct: The treaty of Tordesillas, which was signed on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile.

Incorrect: The treaty which was signed at Tordesillas on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile.

Correct: The treaty that was signed at Tordesillas on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile.

Note that the GMAT almost always tests this rule by incorrectly using which without a comma rather than that with a comma, as in the second set of sentences above.

Why? Because the use of which without a comma is much more difficult for most people to identify as an error. That’s hardly a surprise since that construction is considered perfectly acceptable in everyday writing, particularly in British English. The GMAT, alas, is entirely uninterested in that fact and insists that you adhere strictly to the “only use which after a comma” rule.

Regardless of what you happen to think of that, knowing the GMAT’s preference can help you quickly eliminate answers on questions like this:

The treaty which was signed at Tordesillas on June 7, 1494, and authenticated at Setúbal, Portugal, divided the newly discovered lands outside Europe between Portugal and the Crown of Castile.

(A) The treaty which was signed at Tordesillas on June 7, 1494, and authenticated
(B) The treaty signed at Tordesillas on June 7, 1494, and authenticated
(C) The treaty which was signed at Tordesillas on June 7, 1494, and being authenticated
(D) The treaty of Tordesillas, signed on June 7, 1494, and it was authenticated
(E) The treaty that was signed at Tordesillas on June 7, 1494, its authentication

Sorry, you didn’t think I was going to make things overly straightforward here, did you?

Even if you can’t use the rule we just covered to get all the way to the answer, (B), you can at least cross out (A) and (C) right away. That allows you more room to work carefully through the other answers. (D) and (E) both create awkward and ungrammatical constructions, so they can be eliminated.