1. First decide exactly who or what you mean
A defining relative clause helps identify a person, thing or relevant group. Its content helps answer the question 'which one?'; this clause is not separated by commas.
A non-defining relative clause gives a separate additional fact about a person or thing already sufficiently identified in the conversation. In writing, this clause is separated by commas.
The guests who need a taxi should wait here.
The guests who need a taxi should wait here.
The instruction applies to the group identified by their need for a taxi.
Our guests, who arrived this morning, are resting.
Our guests, who arrived this morning, are resting.
The speaker is referring to already identified guests and adds their arrival time.
2. An additional fact can still be important
'Additional' does not mean 'unimportant' or 'untrue'. Removing a non-defining clause leaves the main message about the same referent, but loses one fact.
Removing a defining clause often leaves a grammatical sentence too. However, it may cover a different group or no longer explain which object to choose. The test 'the sentence is still grammatical' does not determine the type on its own.
The machines which failed the test need repairs.
The machines which failed the test need repairs.
Without which failed the test, the message would apply to all the machines identified by the context.
The lift, which is out of order, is at the end of the hall.
The lift, which is out of order, is at the end of the hall.
The fault is an important fact, but it is presented separately from the information about the location of the already identified lift.
3. Commas mark the clause boundaries
A non-defining clause within a sentence has a comma before and after it. At the end of a sentence, it needs a comma before it, and the sentence ends with the usual full stop or other final punctuation.
Do not automatically put a comma before an English relative word such as which. Defining relative clauses do not need commas.
Known noun, who/which ... , main verb ... | main clause + known noun, who/which ... .
- 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.
Marta, who manages the café, is away today.
Marta, who manages the café, is away today.
The commas enclose who manages the café; the main clause is Marta is away today.
We invited Marta, who manages the café.
We invited Marta, who manages the café.
The additional clause is at the end; do not add another comma before the full stop.
The café that opens at six serves breakfast.
The café that opens at six serves breakfast.
The opening time identifies the relevant café, so there are no commas.
4. Who, which and that are not names for three clause types
In a defining clause, who or that can refer to people, and which or that can refer to things. In a non-defining clause in modern standard writing, use who for people and which for things; that is not suitable here.
Which does not automatically mean non-defining. Restrictive which is standard; some editorial styles, particularly American ones, prefer that. Unless a particular style is specified, both forms are acceptable for things in a defining clause.
The software which tracks expenses is free.
The software which tracks expenses is free.
Which identifies the software being discussed. That is also possible; do not add commas.
BudgetBox, which tracks expenses, is free.
BudgetBox, which tracks expenses, is free.
In this example, the name already identifies the software; its function is presented as an additional fact.
5. When you can omit the relative word
In a defining clause, an object relative pronoun can be omitted: the report I sent. The subject I remains after the omission. Do not omit a subject pronoun from a full relative clause: the report that arrived.
Keep the relative word in a non-defining clause, even when it is an object. Do not duplicate it with it, he, she or them in the same role.
The report I sent contains the figures.
The report I sent contains the figures.
I performs the action sent, and report is its object. That I sent or which I sent is also possible.
Report A, which I sent yesterday, contains the figures.
Report A, which I sent yesterday, contains the figures.
The additional fact requires which; do not add it after sent.
The report that arrived yesterday is incomplete.
The report that arrived yesterday is incomplete.
That is the subject of arrived and cannot simply disappear.
6. Context matters more than counting relatives
My brother who lives in Madrid often distinguishes this brother from others. My brother, who lives in Madrid, presents his place of residence as an additional fact about a brother already identified for the listener.
The commas alone do not prove there is only one brother. Someone may have several brothers but be talking about one already mentioned. A name also often identifies a person, but when two people share the name, a defining specification may be needed. Determine the role of the information in the particular conversation.
My brother, who lives in Madrid, can collect you.
My brother, who lives in Madrid, can collect you.
The listener already knows which brother is meant. The sentence does not establish how many other brothers there are.
The Alex who works upstairs is joining us.
The Alex who works upstairs is joining us.
The context includes several people named Alex; the clause selects the relevant one.
7. Check your own writing
First decide whether the relative clause identifies the referent or adds a fact about it. Then check the comma boundaries, the relative word, and the subject and object roles.
In open exercises, compare your answer with the model and criteria. Different natural wording or a construction with the same meaning may be correct. A grammatical phrase that changes the relevant group or lacks the requested relative clause does not meet the exercise's aim.
Please email the participants who missed the briefing.
Please email the participants who missed the briefing.
The relative clause identifies the recipients; this is not automatically an instruction to email every participant.
Please email the participants, who missed the briefing.
Please email the participants, who missed the briefing.
Here, all the participants being referred to are presented as having missed the briefing. Keep the context and the comma distinction in view to understand the difference.
Common mistakes
Incorrect:Olena, that leads the team, is on holiday.
Correct:Olena, who leads the team, is on holiday.
A non-defining relative clause about a person requires who, not that.
Incorrect:The documents, which I copied them, are on the desk.
Correct:The documents, which I copied, are on the desk.
Which is already the object of copied; them unnecessarily duplicates the same role.
Incorrect:The driver works at night needs a break.
Correct:The driver who works at night needs a break.
The full defining clause works at night needs the relative subject who or that.
Incorrect:The Blue Hall, we booked yesterday, has a piano.
Correct:The Blue Hall, which we booked yesterday, has a piano.
Do not omit which in a non-defining clause, even when it is the object of booked.
Incorrect:Nadia, who owns the shop is here.
Correct:Nadia, who owns the shop, is here.
An additional clause within a sentence must be closed with a second comma before the main verb is.