“Which” and “that” are hard to figure out—if you use the wrong one, your sentence can still be understood, only less clearly. To make things trickier, casually using the wrong one is not only common, but tolerated! But if you want to write clearly and effectively, understanding the difference between “which” and “that” is essential.

Usually it’s obvious whether we want to use “which” or “that.” It only becomes confusing when deciding on whether to use “which” or “that” as a relative pronoun in a relative clause. Let’s explain what that means, and how you can tell when to correctly use “which” and when to use “that.”

Dependent clauses vs. relative clauses

To understand how to choose between “which” and “that” in a relative clause, first we need to know what a “relative clause” even is!

A relative clause is a kind of dependent clause:

A dependent clause is a series of words that has both a subject and a predicate (or verb), just like a sentence. The difference between a dependent clause and a sentence is that a dependent clause supports a sentence but cannot stand alone.

Example of a dependent clause:

Before I ate dinner

This example is a dependent clause because it has a subject (I and a verb (ate), but it’s not a complete sentence—it doesn’t make sense without another clause to complete the thought.

A relative clause is a kind of dependent clause that adds extra information about a noun in a sentence. Relative clauses almost always come right after the nouns they modify, and they always start with a relative pronoun. Can you guess two relative pronouns? That’s right: “that” and “which”!

Example of a relative clause:

Before I ate dinner, which my friend made, I took a nap.

In this example, the highlighted clause is a relative clause. It begins with a relative pronoun (which) and adds additional information to a different clause—in this case, it gives you more information about the dinner you ate (that it was made by your friend, and not by anyone else).

Restrictive clauses vs. nonrestrictive relative clauses

There are two kinds of relative clauses: restrictive and nonrestrictive.

Restrictive relative clauses are relative clauses that are needed to make the sentence complete—they’re essential to the meaning of the sentence. Since the relative clause is essential, it’s not offset by commas.

Example:

I liked the dinner that I ate last night.

In this sentence, the restrictive relative clauses is highlighted. It’s a restrictive clause because the information it conveys is critical to our understanding of the sentence—that of all the dinners I’ve had, the specific dinner I enjoyed was the dinner that I ate last night. Without the restrictive clause, how would we know which dinner we were referring to?

Nonrestrictive relative clauses are relative clauses in which the information conveyed is nice to have, but doesn’t change the meaning of the sentence—the relative clause is non-essential. Nonrestrictive relative clauses are offset by commas.

Example:

I liked the dinner, which wasn’t too heavy.

In this sentence, the nonrestrictive relative clause is highlighted. It’s nonrestrictive because it’s not essential to our understanding of the sentence—it adds more detail about the dinner I liked, but removing the clause wouldn’t change the core meaning of the sentence.

How to know when to use “which” or “that”

Whether you use “which” or “that” depends on whether you’ve written a restrictive clause or a nonrestrictive clause.

When to use "which"

Use “which” to begin a nonrestrictive clause. Remember that nonrestrictive clauses add nonessential information to the sentence, and are always offset with commas.

Example:

My corn patch, which I planted in spring, is ready for harvest.

In the example above, that the corn patch was planted in spring is an embellishment. It’s nice to know, but it doesn’t contain information that you need to know to understand the meaning of the sentence. Therefore the clause is a nonrestrictive clause, and should begin with “which.”

Example:

Carl forgot his lunchbox, which was blue, on the playground at recess.

That the lunchbox is blue is not essential information. If the sentence read, “Carl forgot his lunchbox on the playground at recess,” the meaning would remain the same. Therefore we have another nonrestrictive clause, and those always begin with “which.”

Example:

The flowers, which had grown tall, were bright under the sun.

Again, removing the nonrestrictive clause would not change the meaning of the sentence. Notice how the nonrestrictive clause is offset by commas.

When to use "that"

Use “that” to begin a restrictive clause. Remember that restrictive clauses add essential information to the sentence and are not offset with commas.

Example:

His shirt that has the gravy stain on it needs to be cleaned for the wedding.

In the example above, that has the gravy stain on it is essential information. Without that restrictive clause, the sentence is still grammatically correct, but we wouldn’t know which of his many shirts needs to be cleaned.

Example:

The soda that is cherry flavored is Lucy’s favorite.

In this example, that is cherry flavored is clearly restrictive. Cherry is Lucy’s favorite soda, not all sodas are her favorite.

Example:

My camera that has the long-focus lens is the one I take on camping trips.

“That” is used if you’re indicating a subset of a group, for example, if you’re referring to one camera out of many. If you have many cameras, it’s essential that your readers or listeners know that the camera with the zoom, and not any other camera, is the one you take on the trail.

Breaking the rules of “which” vs. “that”

Now that we’ve laid down the rules of when to use “which” and when to use “that,” it should be mentioned that as writers, it’s sometimes OK to break the rules—and skilled authors often do.

While most writers stick to the rules when using “that”—in other words, only using “that” in restrictive clauses—in modern English it’s not totally unusual to see “which” used in both restrictive and nonrestrictive clauses. So, if you think your prose sounds better with “which,” go for it! You’re in the good company of writers throughout history.

Which vs. that FAQs

Not so difficult once you break it down, is it? Let’s wrap up with some quick and easy which vs. that FAQs.

What’s the difference between a restrictive clause and a nonrestrictive clause?

A restrictive clause is a short section of a sentence—in other words, a kind of relative clause—that adds necessary information to the rest of the sentence. If you were to take the restrictive clause away from the sentence, the meaning of the sentence would change. A restrictive clause will not be separated from the rest of the sentence by a comma.

A nonrestrictive clause is the opposite: it gives new information about the sentence, but that information isn’t necessary to understanding the sentence. The rest of the sentence works just fine without the nonrestrictive clause. A restrictive clause will be set apart from the rest of the sentence by commas.

Can which and that be used interchangeably?

Sometimes they can, but only if the meaning of the sentence doesn’t change. A common mistake new writers often make is using that vs. which incorrectly, which ends up changing the meaning of what they’re trying to say. For example:

Trees that stay green all winter are known as conifers.

Correct. “that stay green all winter” gives us important information we need to understand the idea.

Trees, which stay green all winter, are known as conifers.

Incorrect. Not all trees stay green all winter, and they’re not all known as conifers.

Sometimes you can use “that” and “which” interchangeably and keep the same meaning. You’ll see this more commonly in British English. For example:

She showed him the one that was broken.

She showed him the one which was broken.

Both of these indicate something specific, and both work in the flow of the sentence. You can also use which instead of that to avoid repetition. For example, That which doesn’t kill you makes you stronger. Technically it should be, That that doesn’t kill you makes you stronger, but the phrase isn’t quite as effective that way.

How do you remember the difference between which and that?

It’s helpful to remember “which” as a question: which one? If the sentence is questionable, it’s not necessary. “That” is decisive: that one. That means it’s an important and necessary part of your sentence.

Clauses using “which” will usually have commas around them, and clauses using “that” will not. If you’re not sure, try both and see if the meaning of your sentence changes between one and the other.