Grammar · Punctuation · B1 · Intermediate
Comma before 'which'
Last updated 21 July 2026
In short
What's the difference between defining and non-defining clauses?
A defining (restrictive) clause identifies exactly which person or thing you mean — remove it, and the sentence loses essential meaning. A non-defining (non-restrictive) clause adds extra detail about something already identified — remove it, and the core meaning survives.
This distinction, not the relative pronoun alone, is what decides whether a comma appears. Test it by deleting the clause: if the sentence still names the same person or thing, the clause is non-defining and needs commas.
- The laptop that overheats is the one I returned.(defining — identifies which laptop, no comma)
- My laptop, which overheats constantly, is only two years old.(non-defining — extra detail, commas required)
Do you need a comma before 'which'?
In modern British usage, 'which' almost always introduces a non-defining clause, so it almost always takes a comma before it. 'That' is reserved for defining clauses and never takes a comma.
If you catch yourself writing 'which' with no comma in front of it, check whether 'that' would fit better — that usually signals the clause is actually defining.
- The report, which took two weeks to write, impressed the board.
- The report which took two weeks to write impressed the board.(non-defining 'which' clause needs a comma → The report, which took two weeks to write, impressed the board.)
- The report that impressed the board took two weeks to write.(defining clause with 'that' — no comma)
Does the same rule apply to 'who'?
Yes. 'Who' can introduce either a defining or a non-defining clause about a person, so the comma rule follows the same test as 'which': no comma when the clause identifies the person, commas when it just adds extra detail.
- The colleague who trained me has just been promoted.(defining — identifies which colleague, no comma)
- Mr Rahman, who trained me, has just been promoted.(non-defining — Mr Rahman is already identified by name)
How do you punctuate a non-defining clause in the middle of a sentence?
When the non-defining clause sits inside the sentence rather than at the end, it needs a comma on both sides, like a pair of brackets. Leaving out the second comma is one of the most common punctuation slips at this level.
- The bridge, which was built in 1932, is still the city's main crossing.(comma before and after the inserted clause)
- The bridge, which was built in 1932 is still the city's main crossing.(missing the closing comma → …built in 1932, is still…)
Do you need a comma before 'and', 'but' or 'because'?
The same essential-versus-extra logic extends beyond relative clauses. Before 'and' or 'but', a comma is needed only when a full second clause (its own subject and verb) follows — not when the conjunction simply links two verbs or two short items.
Before 'because', no comma is normally used when the because-clause gives the main reason for the action described. A comma appears only when the because-clause is a separate, additional comment rather than the core reason.
- I revised every evening, and I still felt nervous before the exam.(comma before 'and' — two independent clauses)
- I revised every evening and passed with a high score.(one subject, two verbs — no comma before 'and')
- She left the meeting early because she felt unwell.(no comma — 'because' gives the main reason)
Common mistakes with comma before 'which'
Real learner errors, corrected and explained — the fastest way to stop repeating them.
The hotel that we stayed at, was fully booked next week.
The hotel that we stayed at was fully booked next week.
'That' introduces a defining clause, so it never takes a comma before or around it — the clause identifies which hotel.
My father who taught me to drive, still owns the same car.
My father, who taught me to drive, still owns the same car.
'My father' is already a unique reference (most people have one father), so the clause is non-defining and needs a comma on both sides.
The tunnel which connects the two districts, was closed for repairs.
The tunnel, which connects the two districts, was closed for repairs.
The comma before 'which' was missing — non-defining clauses need commas both before and after.
Employees, who arrive late three times, lose their parking permit.
Employees who arrive late three times lose their parking permit.
This clause identifies which employees the rule applies to, so it's defining and must not be set off with commas.
I missed the bus, because I woke up late.
I missed the bus because I woke up late.
No comma is needed before 'because' when it states the main reason for the main clause.
The engineer designed the bridge, and, tested it personally.
The engineer designed the bridge and tested it personally.
One subject performing two actions joined by 'and' needs no comma at all.
Choose the option, that suits you best.
Choose the option that suits you best.
'That' introduces defining information and is never preceded by a comma.
Comma before 'which' in IELTS
Grammatical Range and Accuracy is 25% of your Speaking and Writing scores — here is where this structure earns marks.
Speaking
- Dropping a natural non-defining clause into a Part 3 answer — 'The policy, which was introduced last year, hasn't changed much' — is exactly the kind of complex structure examiners reward as evidence of a wide range of structures at Band 7+.
- In speech there's no comma to get wrong, but the pause before and after a non-defining clause does the same job — practise the slight pause so the extra information doesn't run straight into the main clause.
Writing
- In Task 2, non-defining 'which' clauses let you add supporting detail without starting a new sentence: 'Public transport, which is heavily subsidised in many cities, remains underused.' Missing the second comma is a frequent accuracy slip examiners notice immediately.
- Confusing 'which' and 'that' — or punctuating 'that' with commas — is flagged under Grammatical Range and Accuracy as a punctuation error, even when the sentence is otherwise well built.
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Related lessons
Frequently asked questions
Do you need a comma before 'which'?
Do you need a comma before 'and'?
Do you need a comma before 'because'?
Do you need a comma before 'but'?
What's the difference between 'which' and 'that'?
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