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

As two of the most common words in the English language, we see them and use them in our writing every day. Sometimes it’s obvious whether we want to use “which” or “that.” It only becomes confusing when deciding which right word to use as a relative pronoun in relative clauses.

Let’s explain what that means, and how you can tell when to correctly use “which” and when to use “that.”

“That vs. which” is a grammar rule that trips up a lot of writers.

What’s the difference between “that” and “which”?

The difference between “that” vs “which” is “that” is used preceding a restrictive clause and “which” is used preceding a nonrestrictive clause. This means that a sentence using “that” will have necessary information to understand its meaning, while a sentence using “which” will have additional information which isn’t necessary, but which expands on its original meaning.

To understand this idea better, let’s look at different types of clauses and how they function in a sentence.

Dependent clauses vs. relative clauses

To understand how to choose between “which” and “that” in relative clauses, first we need to know what these terms even mean!

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 (also called an independent clause) is that a dependent clause supports a sentence but cannot stand alone.

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, also called a descriptive clause, is a kind of dependent clause that adds extra information about a noun in a sentence—in other words, it describes it in a certain way. 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”!

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

In this example, the part in between the commas is a relative clause. It begins with a relative pronoun (which) and adds additional information—in this case, it gives you context 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. These can also be called defining clauses and non-defining clauses. Let’s look at a few examples.

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

I liked the dinner that I ate last night.

In this sentence, the defining clause is “that I ate last night.” 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 clauses, or non-defining clauses are relative clauses in which the information conveyed is useful but doesn’t change the meaning of the sentence—it’s non-essential. Nonrestrictive relative clauses are offset by commas.

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

In this sentence, the non-defining clause is “which wasn’t too heavy.” It adds more detail about the dinner I liked, but removing the clause wouldn’t change the core meaning of the sentence.

(Image: two speech bubbles) Restrictive clause: conveys pivotal information; Non-restrictive clause: conveys optional information.

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

Whether you use “which” or “that” depends on whether you’ve written a restrictive clause (which conveys necessary information) or a nonrestrictive clause (optional information).

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.

Let’s look at the following sentences as examples:

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

In this first sentence above, the corn patch being 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.

The same sentence without the nonrestrictive clause would be “My corn patch is ready for harvest.” It still makes sense and is still a complete idea. Therefore the clause is a nonrestrictive clause, and should begin with “which.”

Here’s another example:

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

In this second sentence, the fact 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.”

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

Again, removing the nonrestrictive clause—“which had grown tall”—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.

Here are a few more examples.

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. Not just any shirt—that one specifically.

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

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

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

“That” is used to distinguish 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.

“That” always begins a restrictive clause.

Breaking the grammar rule 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 in writing 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 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.

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

Quick wrap up:

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. The second sentence is wrong because 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.

“Which one? That one!” can help you remember this picky writing rule.

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 in your writing and see if the meaning of your sentence changes between one and the other.