Put a comma before “which” when the clause it starts could be deleted without changing which thing you are talking about, and leave the comma out when the clause is doing the identifying. “My car, which needs new tires, is in the shop” survives the deletion. “The car which needs new tires is in the shop” does not, because the clause is what tells you which car.
Once you know the answer to “do I need a comma before which,” a second question follows immediately, and it is the one people get wrong far more often: whether the clause also needs a comma at its far end.
The Deletion Test
Cover the “which” clause with your thumb and reread the sentence. If you still know exactly which person or object is meant, the clause is extra and takes commas. If the sentence goes vague, the clause is load-bearing and takes none.
- Extra, so commas: “The Louvre, which sits on the right bank of the Seine, draws crowds all year.” Delete the clause and the Louvre is still the Louvre.
- Load-bearing, so no comma: “The museum which sits on the right bank draws the biggest crowds.” Delete the clause and you have lost track of which museum.
Grammar books call the first kind nonrestrictive and the second restrictive. The labels are worth knowing because style guides use them, though the thumb test settles real sentences faster than the terminology does.
One shortcut fails often enough to warn about: proper nouns and other one-of-a-kind subjects are already fully identified, so a clause after them is almost always the extra kind. “Amsterdam, which sits below sea level” takes the comma; there is only one Amsterdam to specify.
Why American Style Steers You to “That”
In American editing practice, restrictive clauses take “that” and nonrestrictive clauses take “which.” Under that convention, a “which” without a comma reads as an error to be fixed by swapping in “that.”
- American house style: “The car that needs new tires is in the shop.”
- Also American house style: “My car, which needs new tires, is in the shop.”
The rule comes from H. W. Fowler, who proposed the tidy split in 1926 while acknowledging that writers had never observed it. The Chicago Manual of Style recommends it; the Associated Press follows it; academic style guides enforce it. So in US-facing work, treat “comma plus which” and “no comma plus that” as the two approved shapes.
The Error Almost Everyone Makes
An extra clause sitting in the middle of a sentence needs commas on both sides. Writers open the pair and forget to close it, which leaves the sentence hanging.
- Broken: “Her second novel, which took four years to finish sold half a million copies.”
- Fixed: “Her second novel, which took four years to finish, sold half a million copies.”
Think of the pair as brackets. Whatever sits between them can be lifted out cleanly, and a bracket with no partner leaves the reader waiting for a subject that already went past. This is the same paired-comma logic that governs an interrupting “however” or a mid-sentence aside, which is why “however” takes a comma on each side when it interrupts a clause.
When the Clause Ends the Sentence
A closing “which” clause takes one comma at the front and no comma at the back, because the period does the closing.
- “We rescheduled for Tuesday, which suited everyone.”
- “He turned down the offer, which surprised nobody at the table.”
People search for the end-of-sentence case because the missing second comma looks like an omission. It is not. A comma before a period is never correct in English, and the sentence is finished as written.
“Which” Pointing Back at a Whole Idea
Sometimes “which” refers to the entire statement before it instead of to one noun, and that construction is extra by definition, so it always takes a comma.
- “The flight was delayed six hours, which ruined the connection.”
- “She answered every question without notes, which impressed the panel.”
“That” cannot do this job, so no swap is available here. If the sentence grows long enough for the reference to blur, name the thing outright: “the delay ruined the connection.”
“In Which,” “Of Which,” “For Which”
A preposition in front of “which” has no effect on the punctuation, so run the deletion test exactly as before.
| Sentence | Comma? | Why |
|---|---|---|
| The report, in which the figures appear, runs to 90 pages. | Yes, paired | The report is already identified. |
| The year in which she qualified was 2011. | No | The clause specifies the year. |
| Three of the applicants, all of whom had degrees, withdrew. | Yes, paired | Extra detail about a known group. |
The last row shows “whom” behaving identically: person or object, the punctuation follows from the same removability question, and the choice between who and whom sits outside it entirely. Editors who tighten a sentence by cutting the preposition (“the box it came in”) lose the punctuation question along with the preposition, since a bare relative clause of that shape is always the identifying kind.
Where the Rule Loosens: British English
British publishing never adopted Fowler’s split as a hard rule. Restrictive “which” with no comma appears freely in British books, newspapers, and academic prose: “the argument which he makes in chapter three” is standard there, and swapping in “that” is optional.
What British practice keeps is the comma itself. A nonrestrictive clause still takes its pair of commas on both sides of the Atlantic, so the punctuation question has one answer everywhere; only the that-versus-which preference shifts. If you write for a US publication, apply the split. If you write for a UK one, a comma-free “which” needs no correction.
Quick Reference
| Situation | Comma before “which”? | Example |
|---|---|---|
| Clause can be deleted | Yes, and close it too | The bridge, which reopened in May, carries 40,000 cars a day. |
| Clause identifies the noun | No (US style prefers “that”) | The bridge that reopened in May carries the most traffic. |
| Clause ends the sentence | Front comma only | They cancelled the launch, which cost them the quarter. |
| “Which” refers to the whole idea | Yes | He arrived an hour early, which helped. |
| Preposition + which | Apply the deletion test | The box in which it came was damaged. |
Purdue OWL’s comma rules file this under essential and nonessential elements, the same heading that covers appositives and mid-sentence asides.
FAQ: Comma Before “Which”
How do you use “which” in a sentence with a comma?
Place the comma directly in front of “which” when the clause adds detail you could delete, and add a second comma at the end of that clause if the sentence continues afterward: “The lease, which runs to 2029, includes parking.”
What are the main comma rules?
Commas join independent clauses with a coordinating conjunction, separate items in a series, follow introductory elements, bracket nonessential information, set off direct address and quotations, and divide coordinate adjectives, dates, and addresses. Clauses beginning with “which” fall under the bracketing rule.
Does “that” ever take a comma before it?
No, when “that” introduces a relative clause. Such clauses identify the noun, so the sentence needs them and the comma stays out: “the email that arrived at noon.” The absence of a comma is part of what signals the clause is essential.
What is the Oxford comma rule?
The Oxford comma is the optional comma before the final item in a list of three or more. It is a style choice unrelated to relative clauses, and the full rule for commas before “and” covers when it becomes mandatory for clarity.
Two questions, asked in order, settle every sentence: can the clause come out, and does the sentence keep going after it? The first decides whether a comma appears at all; the second decides whether you owe a partner comma at the far end.
