Comma before 'which'
Put a comma before 'which' when it introduces a non-defining relative clause, extra information that could be removed without changing who or what you mean. Use 'that' with no comma for defining (essential) information instead. This one rule also decides whether 'who', 'and', 'but' and 'because' need a comma before them.
Which one do I need?
Comma or no comma?
Every answer at a glance
that, no comma
The clause tells you which thing you mean, so it is defining: use that, with no comma before or around it.
The course that I chose includes a year abroad.
comma + which + closing comma
The clause is extra information in the middle of the sentence, so it needs a comma on both sides, like a pair of brackets.
Our new library, which cost twelve million pounds, opens in March.
comma + which
The clause adds extra information about something already clear, so it is non-defining: put a comma before which.
I applied to Edinburgh, which has an excellent medical school.
commas around who
The person is already identified, often by a name or 'my father', so the who-clause is extra: put commas around it.
My aunt, who lives in Toronto, is visiting us next month.
who, no comma
The clause tells you which person you mean, so it is defining and takes no commas.
Students who submit late lose ten percent of their mark.
comma before and / but
A full second clause with its own subject and verb follows, so a comma before and or but is correct.
The plan was expensive, but it cut journey times by half.
no comma
And or but only links two verbs or two short items, with no new subject, so no comma is needed.
The engineers tested the bridge and approved it for traffic.
no comma
When because gives the main reason for the action in the main clause, no comma goes before it.
Many people cycle to work because parking is so expensive.
comma before because
A comma appears only when the because-clause is a separate, extra comment rather than the core reason.
The shop must be closed, because all the lights are off.
How to use comma before 'which'
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'
Find what is wrong in each one, then open it to check.
1The hotel that we stayed at, was fully booked next week.What's wrong? Show the fix
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.
2My father who taught me to drive, still owns the same car.What's wrong? Show the fix
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.
3The tunnel which connects the two districts, was closed for repairs.What's wrong? Show the fix
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.
4Employees, who arrive late three times, lose their parking permit.What's wrong? Show the fix
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.
5I missed the bus, because I woke up late.What's wrong? Show the fix
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.
6The engineer designed the bridge, and, tested it personally.What's wrong? Show the fix
The engineer designed the bridge and tested it personally.
One subject performing two actions joined by 'and' needs no comma at all.
7Choose the option, that suits you best.What's wrong? Show the fix
Choose the option that suits you best.
'That' introduces defining information and is never preceded by a comma.
Practise
Write your own
Write a sentence of your own with comma before 'which', about you, your studies or your town.
Comma before 'which' in IELTS
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.