Quick answer: That introduces a defining (restrictive) relative clause — essential information that identifies the noun. No commas. Which introduces a non-defining (non-restrictive) clause — extra information that can be removed. Always use commas with which. For a fuller definition, see the Cambridge Dictionary entry for ‘which’.
Comparison Table
| Word | Clause type | Commas? | Example |
|---|---|---|---|
| that | defining (restrictive) | No commas | The book that I recommended |
| which | non-defining (non-restrictive) | With commas | The book, which won an award, |
Using That — Defining Clauses
Use that to introduce a defining relative clause. The clause tells us exactly which person or thing is meant. Without it, the sentence would be unclear or meaningless. Never use commas around a defining clause.
The report that she submitted was excellent. (defines which report)
Give me the pen that works. (identifies which pen)
This is the only path that leads to the summit. (essential identification)
When Only That Will Do
After superlatives, “the only”, “the first”, “the last”, and “all”, English strongly prefers that over which.
It was the best film that I have ever seen.
All that remains is to sign the contract.
Using Which — Non-Defining Clauses
Use which to introduce a non-defining relative clause. The clause adds extra information about a noun that is already clearly identified. Removing the clause does not change the core meaning of the sentence. Always put a comma before which and another comma after the clause (unless it ends the sentence).
The annual report, which covers all departments, is online. (extra info)
My pen, which is blue, is on the desk. (pen already identified)
The summit, which stands at 2,000 metres, takes three hours to reach.
Which Referring to a Whole Clause
Which can also refer back to an entire preceding clause or idea, something that cannot do:
She arrived an hour late, which surprised everyone.
He refused to apologise, which made things worse.
Common Mistakes
✗ The car which I bought last year has been reliable. (no commas → should be that)
✓ The car that I bought last year has been reliable.
Note: if you want to use which here, add commas: “My car, which I bought last year, has been reliable.” (non-defining)
✗ The film, that won the award, was brilliant.
✓ The film that won the award was brilliant. (no commas in defining clauses)
✓ The film, which won the award, was brilliant. (non-defining, commas required)
✗ My teacher, which has been very helpful, explained the rule clearly.
✓ My teacher, who has been very helpful, explained the rule clearly.
Use who (not which) for people in non-defining clauses.
Tips for Getting It Right
- Remove the clause test: if removing it changes the meaning or leaves the noun unidentified, use that. If the sentence still makes full sense, use which with commas.
- Comma test: if you need commas, use which. If there are no commas, use that.
- After superlatives: always use that (“the best / only / first / last” + that).
- For people: prefer who in both defining and non-defining clauses.
- Whole-clause reference: only which can refer to a whole preceding idea (“…, which surprised me”).
Related Pages
Also confusing: Who vs Whom, Its vs It’s, Affect vs Effect. Browse all confusing word pairs →
Mini Quiz: Which or That?
Fill in the blank — click Reveal to check your answer.
1. The car ___ I bought last year has been reliable. (that / which)
2. My car, ___ is a hybrid, uses very little fuel. (that / which)
3. This is the only path ___ leads to the summit. (that / which)
4. The summit, ___ stands at 2,000 metres, takes three hours to reach. (that / which)
Which or That?
Three questions — choose the correct word for each sentence.
1. “The book ___ I recommended is on the shelf.” (defining, no commas)
2. “The book, ___ won an award, is on the shelf.” (non-defining, with commas)
3. Which word always requires commas around its clause?
Frequently Asked Questions
What is the difference between which and that in relative clauses?
“That” introduces a defining (restrictive) relative clause — essential information that identifies exactly which noun you mean. No commas are used. “Which” introduces a non-defining (non-restrictive) clause — extra information that could be removed without changing the main meaning. Commas are always used with “which”.
Can I use which instead of that in a defining clause?
In informal British English, “which” is sometimes used in defining clauses, but this is considered incorrect in formal writing. In American English, the distinction is strictly observed. To be safe in any context, use “that” for defining clauses and “which” (with commas) for non-defining clauses.
Why do I use commas with which but not with that?
Commas signal that the clause is non-defining — it adds optional extra information. A non-defining clause introduced by “which” is parenthetical: it could be placed in brackets or removed entirely. A defining clause with “that” is essential to the sentence meaning, so no commas are used.
Can that refer to people?
Yes. “That” can refer to both people and things in defining clauses: “The student that won the prize”, “The machine that broke down”. However, “who” is preferred for people in formal writing: “The student who won the prize”.
Can which refer to people?
“Which” is used for things, not people. For non-defining clauses about people, use “who”: “My colleague, who has been here for ten years, is retiring.” Using “which” for people is considered an error in standard English.
What is a defining relative clause?
A defining (restrictive) relative clause identifies exactly which person or thing is being talked about. Remove it and the sentence loses essential meaning. Example: “The book that I recommended” — without “that I recommended” we do not know which book. No commas are used.
What is a non-defining relative clause?
A non-defining (non-restrictive) relative clause adds extra information about a noun that is already clearly identified. Remove it and the sentence still makes complete sense. Example: “The annual report, which covers all departments, is online.” The clause adds detail but is not essential.
How do I test whether to use which or that?
Try removing the clause. If the sentence still makes sense and refers to the same thing, the clause is non-defining — use “which” with commas. If removing the clause changes the meaning or leaves the noun unidentified, it is defining — use “that” without commas.
Is which or that more formal?
Neither word is inherently more formal than the other. The choice depends entirely on whether the clause is defining or non-defining. However, using “which” incorrectly in a defining clause (without commas) is considered informal or non-standard in formal writing.
How do which and that appear in IELTS and Cambridge exams?
In IELTS and Cambridge writing exams, correct use of relative clauses demonstrates grammatical range and accuracy. Examiners look for accurate use of “that” in defining clauses (no commas) and “which” in non-defining clauses (with commas). Mixing them up, especially omitting commas with “which”, can reduce your grammatical accuracy score.