Which vs. That (SAT Writing)

This is one of those errors that doesn't show up very often, but when it does, there's no way to figure out the answer without knowing the rule.



At its simplest level, you can think of the rule this way: "which" must always have a comma before it, but "that" never needs a comma before it.

If you want to get a little bit more in-depth, "which" is used only to begin a non-essential clause surrounded by commas, and "that" is used to begin an essential clause not surrounded by commas.

Correct: The treaty that ended hostilities between the two countries was only upheld for several months.

NOT

Incorrect: The treaty which ended hostilities between the two countries, was only upheld for several months.

BUT

Correct: The treaty, which ended hostilities between the two countries, was only upheld for several months.

Why use one version rather than the other? Using "that" indicates that the information in the clause is important to the meaning of the sentence and cannot be removed. Using "which," on the other hand, indicates that the information in the clause is not absolutely necessary.