Restrictive Clauses: Use "That"
A restrictive clause defines or limits the noun it modifies. Without it, the sentence refers to a different thing — or becomes meaninglessly vague. Restrictive clauses are not set off by commas.
- The report that contains the updated figures is on my desk. (not any report — specifically the one with the updated figures)
- The policy that was revised last quarter takes effect next month. (a specific policy)
- She chose the candidate that had the most relevant experience. (not any candidate — a specific one)
- Give me the folder that I left on the table. (a specific folder)
Remove the clause and the meaning changes: The report is on my desk no longer tells us which report. The restrictive clause was load-bearing.
Non-Restrictive Clauses: Use "Which"
A non-restrictive clause adds information about a noun that has already been sufficiently identified. It could be removed without changing what the sentence is about. Non-restrictive clauses are set off by commas — one before which and one at the end of the clause (or a period if it ends the sentence).
- The annual report, which was released on Tuesday, exceeded expectations. (there is only one annual report; the date is extra information)
- The policy, which was revised last quarter, takes effect next month. (we already know which policy; the revision fact is supplemental)
- Her first novel, which won a national award, was published in 2018. (she has one first novel; the award is added detail)
- The conference room, which seats thirty people, will be reserved for the presentation.
Remove the clause and the sentence still identifies the same thing: The annual report exceeded expectations. The non-restrictive clause was informative, not defining.
The Comma Test
Commas and the which/that choice go together:
- No comma before the clause → restrictive → use that.
- Comma before the clause → non-restrictive → use which.
If you're unsure which to use, ask: "Can I remove this clause and still mean the same specific thing?" If yes, the clause is non-restrictive: use which with a comma. If removing the clause makes the noun unidentifiable or changes the meaning, the clause is restrictive: use that with no comma.
Side-by-Side Examples
| Restrictive (that, no comma) | Non-Restrictive (which, comma) |
|---|---|
| The car that has the dent is mine. (identifies which car) | My car, which has a dent, needs repair. (my car is already identified) |
| The clause that governs liability is on page twelve. (one specific clause) | The liability clause, which was added in 2023, governs damages. (already identified) |
| A drug that reduces inflammation is being tested. (any drug with that property) | The drug, which reduces inflammation, was approved last year. (a specific drug) |
Why the Distinction Matters
In some sentences, using which vs. that (and adding or omitting a comma) changes the meaning significantly:
- The offices that were renovated are on the third floor. — Only the renovated offices are on the third floor; the others may be elsewhere.
- The offices, which were renovated, are on the third floor. — All the offices (which happen to have been renovated) are on the third floor.
The first sentence implies there are multiple groups of offices. The second implies all offices are being discussed as a single group.
When "Which" Can Be Used Without a Comma
In British English, which is commonly used in both restrictive and non-restrictive clauses. The book which I recommended is out of stock is standard in British usage. In American English, most style guides (Chicago, AP, Garner's) call for that in restrictive clauses. If you write for a primarily American audience or follow American style guides, maintaining the that/which distinction is expected.
After Prepositions: "Which" Is Required
When the relative pronoun follows a preposition, which is used regardless of whether the clause is restrictive:
- The topic about which we were arguing was never resolved. (formal; restrictive)
- The city in which she grew up no longer exists.
In informal writing, speakers tend to move the preposition to the end and use that or drop the pronoun entirely: The topic we were arguing about or The city she grew up in. Both approaches are common; the fronted preposition with which is more formal.
Common Errors
- Using which without a comma in a restrictive clause: The document which you requested is attached — should be that you requested in American English, or add commas if it is non-restrictive.
- Using that after a comma: The document, that was revised last week, is ready — that cannot follow a comma in a relative clause. Use which.
- Using that for people: When the relative clause modifies a person, use who rather than that in formal writing. The employee who submitted the report is preferred over the employee that submitted the report, though the latter is accepted in informal contexts.
Frequently Asked Questions
Can "that" introduce a non-restrictive clause?
No. That cannot introduce a non-restrictive clause in standard English. Non-restrictive clauses are always introduced by which (for things) or who/whom (for people), and they are always set off by commas.
Is the which/that rule strictly enforced?
In formal American English — academic, legal, journalistic — yes. The Chicago Manual of Style and most major American style guides enforce it. In British English, which is used in both clause types. In casual writing and speech, the distinction is frequently ignored without causing misunderstanding. The stakes rise when the two possible interpretations produce different meanings, as in the offices example above.
What about "that which"?
That which is a formal construction meaning "the thing that": That which cannot be measured cannot be managed. It is a fixed phrase where that is a demonstrative pronoun and which introduces the following clause. This is not the same as choosing between which and that as relative pronouns.