That vs. Which: Restrictive and Nonrestrictive Clauses Explained
The comma changes everything. Here is how restrictive and nonrestrictive clauses pick the right word.
A Pair Decided by a Comma
That and which both introduce clauses that describe a noun, and most writers treat them as interchangeable. They are not, and the difference comes down to one comma. In American English, 'that' introduces a clause you cannot remove without changing the meaning, while 'which' introduces one you can remove. The comma signals which kind of clause you have.
The two clause types have names worth knowing. A restrictive clause limits the noun it modifies, narrowing it down, and you cannot drop it without losing essential information. A nonrestrictive clause adds extra detail that is nice to have but not essential, and it sits between commas. Once you sort clauses this way, picking that or which becomes mechanical.
Restrictive vs. Nonrestrictive Clauses
| Feature | Restrictive clause | Nonrestrictive clause |
|---|---|---|
| Function | Narrows down the noun, essential | Adds extra detail, optional |
| Commas | No commas | Set off by commas |
| Usual relative pronoun (US) | that | which |
| Can you remove it? | No, meaning changes | Yes, meaning stays intact |
The Restrictive Clause: That
A restrictive clause identifies which specific thing you mean. In 'the car that is parked in the driveway needs a wash', the clause 'that is parked in the driveway' tells us which car, so it is essential. Remove it and the sentence becomes 'the car needs a wash', which leaves the reader guessing which car. Because the clause restricts the noun, it takes no commas and, in US English, uses 'that'.
Restrictive clauses matter because they carry identifying information. 'The students who passed the exam will get certificates' uses 'who' to restrict the group to a specific set of students. Swap in 'which' or add commas, and the sentence either sounds wrong or shifts meaning. When the clause is doing the work of narrowing down, treat it as restrictive.
In British English, 'which' is often used for both restrictive and nonrestrictive clauses, and the comma does the work alone. American English is stricter, reserving 'which' for nonrestrictive clauses and 'that' for restrictive ones. If you write for a US audience, follow the stricter rule.
The Nonrestrictive Clause: Which
A nonrestrictive clause adds information that is true but not essential to identifying the noun. In 'my car, which is ten years old, still runs well', the clause 'which is ten years old' is extra detail. You could remove it and the sentence still identifies the car. Because the clause is optional, it sits between commas and uses 'which'.
The comma test is the practical shortcut. If the clause needs commas, use 'which'. If it does not, use 'that'. The rule holds across most sentences in American English, and it removes the guesswork. Try reading the sentence without the clause: if the meaning stays the same, you have a nonrestrictive clause and the comma-plus-which pattern applies.
Common Slip-Ups to Avoid
- Using 'which' without commas for a restrictive clause, common but flagged in US editing
- Adding commas around a clause that is actually essential to identifying the noun
- Treating 'that' and 'which' as fully interchangeable, which they are not in US English
- Forgetting that the rule for people uses 'who', not 'that' or 'which', in careful writing
Choosing That or Which in Four Steps
- 1Test whether the clause is essential
Read the sentence without the clause. If the meaning changes or the noun becomes unclear, the clause is restrictive.
- 2Check for commas
If the clause is set off by commas, it is nonrestrictive and takes 'which'. If there are no commas, it is restrictive and takes 'that'.
- 3Confirm your variety of English
In American English, keep 'that' for restrictive and 'which' for nonrestrictive. British English allows more flexibility, but be consistent.
- 4Use 'who' for people
When the noun is a person, switch to 'who' regardless of clause type, since careful readers expect it over 'that' or 'which'.
The Rule in Brief
If you need the clause to know which one, use that with no commas. If you could drop it and still know, use which between commas.
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