Use that for defining (essential) information, with no comma (the book that I bought). Use which for extra, non-essential information, set off by a comma (the book, which I bought yesterday, is excellent).
The Core Difference
“That” introduces a defining (restrictive) relative clause — information essential to identifying what is being talked about, never separated by a comma: the phone that I lost was new.
“Which” introduces a non-defining (non-restrictive) clause — extra information that could be removed without changing the core meaning of the sentence, always set off by a comma (or commas): my phone, which I lost yesterday, was new.
- essential info, no comma → that: the car that I drive is old
- extra info, with comma(s) → which: my car, which is old, still runs well
Which and That in Context
The laptop that crashed yesterday is now fixed.
My laptop, which is three years old, still works fine.
This is the restaurant that everyone recommends.
The report, which took two weeks to write, was well received.
Choose the option that best suits you, which, in most cases, is the cheaper plan.
Common Mistakes
| Incorrect | Correct | Why |
|---|---|---|
| ✗ the car, that I drive, is old | ✓ the car, which I drive, is old | non-essential info needs which+comma |
| ✗ the phone which I lost was new | ✓ the phone that I lost was new | essential info, no comma, prefer that |
| ✗ my house that is old needs paint (if only one house) | ✓ my house, which is old, needs paint | extra info about the only house |
| ✗ she bought a car which broke down (defining) | ✓ she bought a car that broke down | essential/defining clause |
| ✗ the book that, I mentioned earlier, is here | ✓ the book, which I mentioned earlier, is here | non-defining needs commas |
THAT = no comma, essential (Tightly attached). WHICH = with comma(s), extra (Withdraw-able) — if you can delete the clause without losing key meaning, it takes “which” and commas.
Related Questions
- Which vs That — full confusing-words comparison page with additional examples and exercises.
- All Confusing Words — browse all 300+ word-pair guides.
- All Questions — more quick-answer pages.
- Grammar Quiz — test your knowledge of confusing word pairs.
Frequently Asked Questions
What is the comma test for which vs that?
If removing the clause would change the sentence's essential meaning, use “that” with no comma. If the clause is just extra information that could be deleted, use “which” with commas.
Can which ever be used without commas?
In American English style guides, “that” is strongly preferred for defining clauses and “which” for non-defining ones with commas. British English is more flexible and sometimes uses “which” without commas in defining clauses, though “that” remains the safer, clearer choice.
Is that used for people as well as things?
Traditionally “that” is used for things and “who” for people, though “that” is commonly used informally for people too (“the man that called”). Formal writing prefers “who” for people.
Why do commas matter so much with which?
Because a non-defining clause with “which” is optional extra information, the commas act like verbal parentheses, signalling that the sentence's core meaning stands without it.
Can removing which change the meaning of a sentence?
Yes, if you remove a non-defining “which” clause, the essential meaning stays the same, only losing extra detail. Removing a defining “that” clause, however, can make the sentence unclear or lose its identifying information.
Is this distinction important in formal writing and exams?
Yes — correctly punctuating which/that clauses is a common marker of writing accuracy in IELTS, academic essays, and professional documents.
Does British English follow the same rule as American English?
Broadly yes, though British English is historically more relaxed about using “which” in defining clauses without a strict rule; American style guides (like AP and Chicago) enforce the that/which distinction more strictly.
What is a simple example showing the meaning difference?
“The employees that finished the training received a certificate” (only those who finished) versus “The employees, which finished the training, received a certificate” (implying all employees finished) — the choice changes what is being claimed.
Why do formal phrases like in which or for which use which even for defining clauses?
After a preposition, only “which” can be used, not “that”: “the method in which the test was conducted” cannot become “in that”. This is one case where “which” appears in a defining clause.
Are there words that force the use of that instead of which?
Yes — after superlatives and words like “all”, “only”, “the same”, and “the first/last”, English strongly prefers “that”: “the best film that I've seen” rather than “which”.