Which vs. That: A Simple Rule for Relative Clauses (Plus the Grammar Behind It)
Mixing up which and that is one of the most common small errors in writing — but it’s also one of the easiest to fix once the idea clicks. In short, it comes down to whether the clause is restrictive or nonrestrictive. Once you spot which one you’re dealing with, the right word follows automatically.
So here’s the full rule, along with the grammar behind it and the punctuation that trips most people up.
How to Use Which vs. That: The 10-Second Rule
First, try this on your sentence:
- Still the same use which
- Meaning changed use that
Example:
In short, that’s really the whole rule — everything below just fills in the details and the exceptions.
What’s a Relative Clause? A Fast Grammar Refresher
Put simply, a clause is a group of words built around a subject and a predicate. A dependent clause, however, can’t stand on its own as a full sentence — it needs the rest of the sentence to make sense. In this case, the clauses we’re focused on are a specific kind of dependent clause called relative clauses: they add information about a noun, and they’re introduced by a relative pronoun — usually that, which, or who.
In fact, every relative clause is either restrictive or nonrestrictive, and that’s the only distinction that decides your word choice.
Restrictive Clauses: When “That” Points to One Specific Thing
A restrictive clause narrows down exactly which one you mean, out of a larger group. So if you take it out, the sentence loses the information that makes it clear.
That is the word that typically introduces this kind of clause — sometimes called a defining clause, because it defines which thing you mean. So, no comma, ever. If you catch yourself reaching for a comma before that, that’s usually a sign the clause isn’t actually restrictive, and which is what you want instead.
Nonrestrictive Clauses: When “Which” Adds Extra Detail
A nonrestrictive clause, by contrast, adds a detail about something you’ve already identified. Take it out, and the sentence still says exactly what it said before.
Which introduces this kind of clause — sometimes called a non-defining or descriptive clause, since it describes rather than defines. Essentially, think of it as a side comment, not a definition.
Comma Rules for Which and That
This is where the restrictive/nonrestrictive distinction becomes visible on the page. Specifically, a nonrestrictive clause is always set off by commas — one before it, and one after too if it lands in the middle of a sentence. A restrictive clause, on the other hand, is never surrounded by commas; it flows straight into the sentence with nothing marking it off. (You’ll also see this spelled non-restrictive clause in some style guides — same idea, different hyphenation.)
| That (restrictive) | Which (nonrestrictive) | |
|---|---|---|
| Comma | Never | Always |
| Can you delete it? | No | Yes |
| Job in the sentence | Narrows down meaning | Adds a bonus detail |
Which vs. That: How British and American English Differ
If you read British writing, you’ll notice which used the way Americans use that — narrowing meaning, no commas. That’s not an error, though; it’s a genuine regional difference. In British English, in fact, that and which are often used interchangeably this way. American style guides (AP, Chicago, APA) enforce the stricter split, however, especially in formal writing. So if you’re writing for a U.S. audience, stick to the strict rule.
The distinction as we know it today was popularized by the Fowler brothers, Henry and Francis, in their 1906 style guide The King’s English, now free to read in full. Ultimately, though, it’s a style-guide preference, not an unbreakable law of the language.
Relative Pronoun Basics: That, Which, and Who
That, which, and who are all relative pronouns, and essentially, they follow the same restrictive/nonrestrictive logic — the only difference is what they refer to.
| Refers to | Restrictive | Nonrestrictive |
|---|---|---|
| People | that or who | who |
| Things, animals, groups | that | which |
The candidate who impressed us most got the offer and the candidate that impressed us most got the offer are both correct for people; that said, who just tends to sound more natural.
Common Which vs. That Mistakes
- Comma before “that”: The report, that you sent, has an error → so, drop both commas.
- Missing comma before “which”: Our office which opened in 2019 is downtown → instead, add one before which and one after.
- “Which” doing “that”‘s job: I need a phone which has a better camera → here, swap in that, since you’re narrowing down the kind of phone.
- Exception — after a preposition: which is standard even in a restrictive clause after a preposition: the topic on which we agreed, not “on that we agreed.”
Which vs. That Quiz: Test Yourself
- The contract we signed last week needs revisions.
- My car, is ten years old, still runs great.
- Any employee arrives late will be marked absent.
- The museum, opened in 1920, is downtown.
Which vs. That FAQs
What’s the difference between restrictive and nonrestrictive clauses?
A restrictive clause narrows down which specific thing you mean and can’t be removed without changing the sentence’s meaning. A nonrestrictive clause, by contrast, adds a removable extra detail and is always set off by commas.
Are which and that interchangeable?
Not in careful American writing. In fact, getting them backward can quietly change your meaning or leave you with a punctuation error.
What’s the fastest way to check my own writing?
First, search your draft for every that and which, then ask whether the clause is restrictive or nonrestrictive. If you’re still unsure, the delete test settles it in seconds.
What is the rule for using that or which?
Use that when the clause is essential to the sentence’s meaning, with no comma. Use which, on the other hand, when the clause adds extra, removable detail, always set off by a comma. In short, if deleting the clause changes what the sentence means, it’s that; if the sentence still makes sense without it, it’s which.
Where do we use which?
Which is used to introduce a nonrestrictive clause — one that adds a bonus detail about something already identified, rather than narrowing down which one you mean. It’s always preceded by a comma, and the clause it introduces could be deleted without changing the sentence’s core meaning. In addition, it’s also the standard choice right after a preposition, even in a restrictive clause (the topic on which we agreed).
Which or that examples sentences?
Restrictive (that): The laptop that has a cracked screen is mine. Nonrestrictive (which): My laptop, which has a cracked screen, is on the desk. For another pair, compare Employees that clock in late lose their bonus versus My coworker, which isn’t right — it should be who, since she’s a person, clocks in late every day. When in doubt, try deleting the clause: if the sentence still means the same thing, you want which.
Can you explain the difference between “which” and “that” in simple terms?
That points to one specific thing out of a group — it’s doing real work in the sentence. Which, by comparison, just adds a side comment about something you already know. So, if you can’t delete the clause without losing the point of the sentence, use that. But if you could delete it and lose nothing but a nice-to-know detail, use which — and put a comma in front of it.