A comma signals meaning
In a relative clause, punctuation has a grammatical role. Without commas, the clause answers “which one?” and helps identify the noun. With commas, it does not identify it: the referent is already established and the clause adds a detail.
The same words with and without commas therefore make different claims. This is not simply taste: punctuation expresses whether the information is needed for identification.
Without commas → identify a noun; with commas → add supplementary information.
- noun —
- A word naming a person, thing or idea: student, book, happiness.
My colleague who speaks Japanese is joining the call.
My colleague who speaks Japanese is joining the call.
In this scenario, the clause identifies the Japanese-speaking colleague among several. Removing it would leave the intended colleague unclear.
My colleague, who speaks Japanese, is joining the call.
My colleague, who speaks Japanese, is joining the call.
Commas present the colleague as already identified; speaking Japanese is a supplementary detail that can be removed.
Defining relative clauses: no enclosing commas
A defining clause narrows the reference: it resolves which person or thing is meant. It is attached directly to the noun without enclosing commas.
Pronoun choice is broad: who or that for people, which or that for things. British usage readily accepts defining which; some American editorial guidance reserves which for clauses with commas. The commas work alike in both varieties, so this lesson focuses on punctuation.
Noun + who/that for people or which/that for things + …, without enclosing commas.
- noun —
- A word naming a person, thing or idea: student, book, happiness.
The email that arrived this morning has the new prices.
The email that arrived this morning has the new prices.
Among multiple emails in this context, the clause selects the relevant one without commas.
The people who signed up early get a discount.
The people who signed up early get a discount.
The discount applies to the group identified by the clause rather than an unrestricted group of people.
The room which we booked has no window.
The room which we booked has no window.
Defining which is normal in British usage; that also fits, and neither version needs commas.
Non-defining relative clauses: set off with commas
When a person or thing is already identified and a separate detail is added, set the relative clause off with commas. A proper name or my/our often helps identify a referent but does not determine punctuation automatically: context still matters.
Removing the clause preserves the core message while losing a detail. This is a useful check before adding commas.
Noun + , who/which … , + remaining main clause.
- noun —
- A word naming a person, thing or idea: student, book, happiness.
- clause —
- A construction with a verb, often with its own subject: when I arrived. A non-finite clause may leave the subject unspoken: to get home.
Our new office, which opened in March, is near the river.
Our new office, which opened in March, is near the river.
The participants know which office is meant; March’s opening is an extra detail. Our alone does not guarantee this reading in every sentence.
Mrs Adams, who taught me maths, retired last year.
Mrs Adams, who taught me maths, retired last year.
The name identifies the person in this context; the clause adds information.
Why supplementary clauses do not use relative that
Do not use relative that in a comma-separated supplementary clause. People can take who or object whom; things take which. Whose, where and when can also fit their respective roles.
That is familiar in defining clauses, which can lead to mistakes. For this relative-clause pattern, a comma introducing supplementary information rules out relative that.
, who … , · , which … , · never relative , that … ,
Vienna, which we visited in spring, is easy to walk around.
Vienna, which we visited in spring, is easy to walk around.
The proper name identifies the city; the supplementary clause takes commas and which.
The van that blocked our gate has gone.
The van that blocked our gate has gone.
The clause identifies the van without commas, so relative that is appropriate.
One comma or two?
A supplementary relative clause inside a sentence needs a comma at each end: one opens it, the other returns to the main clause. Omitting the closing comma is a common punctuation error here.
If the clause ends the sentence, final punctuation replaces the need for a separate closing comma. If it is inside the main clause, close it with a comma. This concerns that clause’s boundaries rather than the total number of commas in the sentence.
Noun , relative clause , main verb when internal; … , who/which … . when final.
- verb / V —
- A word for an action or state, such as work or be. The pattern determines its form.
- noun —
- A word naming a person, thing or idea: student, book, happiness.
- clause —
- A construction with a verb, often with its own subject: when I arrived. A non-finite clause may leave the subject unspoken: to get home.
The recipe, which I found in an old notebook, takes ten minutes.
The recipe, which I found in an old notebook, takes ten minutes.
The internal supplementary clause closes with a second comma before main verb takes.
I finally called Halyna, who had left three messages.
I finally called Halyna, who had left three messages.
The relative clause is final, so it needs no second comma.
What makes a noun already identified?
The intended meaning in context determines the choice. If my mother, our landlord or a name identifies the person unambiguously, a relative clause can add a comma-separated detail. But our landlord who lives upstairs can distinguish one of several landlords; our does not prohibit a defining clause.
Plural nouns allow both patterns. The students who arrived early selects the early arrivals; The students, who arrived early, adds that detail about the entire already identified group. Do not choose punctuation merely from the noun’s number.
Need to narrow noun reference → no commas; add a detail about an identified group → commas.
- noun —
- A word naming a person, thing or idea: student, book, happiness.
Lake Como, which is in northern Italy, gets crowded in July.
Lake Como, which is in northern Italy, gets crowded in July.
The name identifies the lake here, so the clause adds information.
The students who missed the deadline have to resubmit.
The students who missed the deadline have to resubmit.
The claim concerns the specified subset rather than unrestricted students, so there are no commas.
When a comma changes the facts
Without commas, a relative clause establishes the group being discussed. With commas, it attributes a property to the already named group. A defining clause alone does not prove that other people necessarily exist outside the group.
Before adding commas, check whether the property actually applies to everyone being discussed. Otherwise, commas can make the claim factually false.
The cyclists who wore helmets finished the race.
The cyclists who wore helmets finished the race.
The sentence reports the finishers who wore helmets. It does not establish whether there were helmetless cyclists or what happened to them.
The cyclists, who wore helmets, finished the race.
The cyclists, who wore helmets, finished the race.
The identified cyclists all wore helmets and all finished. Changing commas changes the claim.
Whole-clause which, pronunciation and scope
After a comma, which can refer to the entire preceding proposition rather than one noun, commenting on the event as a whole. Relative that cannot do this; the comma is required.
In speech, supplementary clauses normally form a separate intonation group with pauses, while defining clauses are integrated. Different readings of the same wording can convey different information.
This lesson does not practise choosing who/which by noun type, omitting pronouns or placing prepositions within a relative clause. Those are related lessons. Here the axis is punctuation and its consequences.
The last bus left early, which meant a long walk home.
The last bus left early, which meant a long walk home.
Which comments on the whole preceding event rather than one word; a comma is required.
She missed the last train, which is why she stayed at ours.
She missed the last train, which is why she stayed at ours.
Which is why also comments on the preceding proposition.
Ask the driver, who knows the way.
Ask the driver, who knows the way.
A pause before the clause signals extra information. Without the comma and pause, the clause would identify the driver who knows the way, potentially among several.
Common mistakes
Incorrect:Vienna, that we visited in spring, is easy to walk around.
Correct:Vienna, which we visited in spring, is easy to walk around.
The clause is set off with commas, so relative that does not fit. Use which for the city.
Incorrect:My aunt, who lives in Cork is arriving tonight.
Correct:My aunt, who lives in Cork, is arriving tonight.
The opening comma lacks its closing partner. Add a second comma before the main-clause verb.
Incorrect:Our landlord who is very strict, checks the flat every month.
Correct:Our landlord, who is very strict, checks the flat every month.
The existing comma only closes the clause. If strict adds a detail about an identified landlord, add the opening comma. To identify one of several landlords instead, remove the existing comma. Our alone does not settle the meaning.
Incorrect:Dr Hales, that runs the clinic, answered my email.
Correct:Dr Hales, who runs the clinic, answered my email.
After the comma, use who for a person; relative that is restricted to defining clauses without enclosing commas.
Incorrect:I need a plumber, who can come today.
Correct:I need a plumber who can come today.
The intended meaning seeks any plumber able to come today, so the clause states a requirement without a comma. A context with an already identified plumber would need a different reading.
Incorrect:He missed the deadline, that annoyed his manager.
Correct:He missed the deadline, which annoyed his manager.
The clause comments on the entire preceding proposition; only which fits this relative-pronoun role.