How to use commas: Using “that” vs. “which”

Last week we talked about using commas to separate the main part of a sentence from nonessential information, information that we can remove from a sentence without altering the meaning of the sentence. Today we’re going to talk about one specific set of cases where we have to decide whether we’re writing about nonessential versus essential information: when we’re deciding whether to use “which” vs. “that” in a sentence.

“Which” is a word that we use when we want to provide information that adds more detail but isn’t essential to the meaning of a sentence. For example, let’s imagine that we’re at a party and we want to tell a friend who has celiac disease that all of the cookies at the party are gluten free. We might write the following sentence:

• “The cookies are gluten free.”

Gluten-free cookies are usually trickier to bake and more expensive to buy than regular cookies, so let’s say that we want to give credit where credit is due and tell our friend that Emily brought the cookies. In this case, we might write a sentence like this:

• “The cookies, which Emily brought, are gluten free.”

In this sentence, “which Emily bought” is nonessential information because we don’t need it in the sentence to know which cookies at the party are gluten free (they all are!). It does provide additional information by telling us who brought the cookies, but it doesn’t play a role in identifying which cookies we were talking about. In other words, removing “which Emily bought” wouldn’t alter the meaning of the main part of the sentence (i.e., “The cookies are gluten free”) and it doesn’t restrict the category of cookies at the party that we’re talking about (which is why “which Emily brought” is called a nonrestrictive clause). When we’re working with nonessential information, we use “which” instead of “that.” We also place a comma before “which.”

In comparison, “that” is a word that we use when we need to provide information that’s essential to the meaning of a sentence. For example, let’s imagine that only some of the cookies at the party are gluten free: the ones that Emily brought. If we want to tell our friend with celiac disease which cookies are safe for her to eat, we might write a sentence like this:

• “The cookies that Emily brought are gluten free.”

In this sentence, “that Emily bought” is essential information because we need it in the sentence to know which cookies are gluten free; there are different types of cookies at the party and it’s only the ones that Emily brought that are gluten free. If we removed “that Emily brought” from the sentence, we would end up implying that all of the cookies are gluten free, even though it’s only the ones that Emily brought that are gluten free. In other words, “that Emily brought” restricts the category of cookies at the party that we’re talking about, which is why “that Emily brought” is called a restrictive clause. When we’re writing clauses that contain essential information, we use “that” instead of “which.” We also don’t use a comma before “that.”

As you can see, the decision about whether to use “that” vs. which” isn’t a trivial or arbitrary one. Instead, it depends on what you’re trying to say to your reader. If you’re trying to provide information that’s important for defining exactly who or what you’re talking about, use “that” and leave the comma out. If you’re providing information that simply gives the reader more details without changing who or what you’re talking about, use “which” and place a comma before it.

Have any lingering questions about when to use “that” vs. “which”? Leave us a note in our comments section below and we’ll do our best to incorporate your questions into one of our upcoming posts.

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