When should you use “that” and when should you use “which”?

The ten-second answer:

“Which” needs a comma in front of it. If it doesn’t have one, you probably need to change it to “that,” or (rarely) need to add a comma in front of it.

Trust your comma-sense. Or your common sense, whichever works better.

The full answer:

“That” and “which” are relative pronouns. A pronoun refers to or takes the place of a noun; a relative pronoun does so in order to introduce a dependent clause that modifies an antecedent noun.

Let’s take the questionable sentence “The summary judgment motion which was filed on Thursday addresses this issue.” “Motion” is the antecedent noun, and “which was filed on Thursday” is a dependent clause.

The clause could be one of two things: restrictive—meaning that it provides crucial defining information about the antecedent, or nonrestrictive—meaning that it just gives us some additional interesting information.

A restrictive clause takes “that,” and no comma, a nonrestrictive clause “which,” and a comma.

So if we want to tell the judge that the summary judgment motion addresses the issue, and by the way, judge, we filed that on Thursday, then it’s a nonrestrictive clause, and the “which” is correct but the clause needs to be set off with commas, before the which and after “Thursday.” If we want to tell the judge that the particular summary judgment motion that addresses the issue is the one that was filed on Thursday, we need “that” and no comma.

Figuring out whether your clause conveys essential information or nonessential, and thus whether it needs a comma, is for most people much easier than deciding whether “which” or “that” is correct.

I want to learn this forever:

Witches always ride commas.

Vividly picture a witch riding a comma across the sky, cackling wildly. She is non-restrictive, calling out interesting but non-essential information.

More information:

There is actually a good deal of debate about the rule given above for that and which, because it is not followed by great writers. It is not at all hard to find “violations” of the rule. For example, you don’t get past the first page of George Orwell’s 1984 without finding this:

On each landing, opposite the lift-shaft, the poster with the enormous face gazed from the wall. It was one of those pictures which are so contrived that the eyes follow you about when you move.

That was, by the way, the very first book that I checked. Here’s a link to many more examples of violations of the that/which rule on the first page of great books.

When you are writing fiction, feel free to follow the lead of Orwell and many other fine writers if you like. But when you are writing for judges and law clerks who may well follow a firm that/which rule, you’re better off following it.

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