Non-defining relative clauses

A defining relative clause answers which person or thing? A non-defining relative clause adds extra information about someone or something already identified.

The students who arrived early found seats.
The clause identifies which students.

The students, who arrived early, found seats.
The sentence presents the students as a known group and adds that they arrived early.

The commas are part of the meaning. Removing the clause from a non-defining sentence should leave a complete main message: The students found seats.

The commas tell us which information identifies the noun

Imagine you have two brothers. One lives in Leeds:

My brother who lives in Leeds is visiting.

Who lives in Leeds identifies which brother. Now imagine that your reader already knows the brother you mean:

My brother, who lives in Leeds, is visiting.

The clause supplies an additional fact about that already identified person. This version does not logically prove you have only one brother; it presents the intended brother as identifiable from the context.

Compare a notice to staff:

Notice	Who must attend?
The staff who work upstairs must attend.	The upstairs group
The staff, who work upstairs, must attend.	The identified staff group as a whole; the clause adds where they work.

A useful reading test is to remove the clause. My brother is visiting is grammatically complete in both examples. The real question is whether your reader can still identify which brother without the extra words. Grammatical completeness alone does not decide whether a relative clause is defining.

Commas come in pairs when the clause is in the middle

Write Nora, who teaches biology, is leading the visit. The second comma closes the additional information before the main verb is leading. Leaving it out creates an unfinished interruption.

At the end of a sentence, only the opening comma is needed: We visited Nora, who teaches biology. The full stop closes the sentence.

Choosing the relative word

Use who or whom for people, which for things and whose for possession:

Maya, who works in the library, recommended the book.
The report, which was published in June, is available online.
Dr Evans, whose research focuses on sleep, gave the talk.
Professor Lane, whom I met at the conference, emailed me later.

Whom is formal and appears especially after a preposition or as an object: the researcher to whom I spoke. In everyday English, many speakers use who or put the preposition at the end: the researcher who I spoke to. Both are understandable; choose a level of formality that suits your writing.

A non-defining clause normally cannot use that and cannot omit its relative pronoun:

The report, which was published in June, is online.
Not: ~~The report, that was published in June, is online.~~

In a defining clause, that may be possible and an object pronoun can sometimes be omitted: The report (that) Maya wrote. That omission does not carry over to non-defining clauses.

Place, time and whole-clause which

For a place, where can introduce additional information: The garden, where we first met, is now a café. For a time, use when: June, when the course begins, is usually busy. More formal versions are possible with prepositions: the building in which we met.

Sometimes which refers to the whole idea in the previous clause, not just one noun:

The company delayed the launch, which disappointed its customers.

This means that the delay disappointed them. The comma helps show that which comments on the whole preceding event. Do not use what in this pattern: what means roughly “the thing that” and does not refer back to a full earlier clause.

Commas with names and titles

A name often identifies the person already, so additional information is set off with commas: Our director, who joined in 2022, will speak first. But punctuation depends on whether the clause identifies someone: My colleague who joined in 2022 will speak first can imply that I have more than one colleague and mean the one who joined in that year.

Cambridge explains this information distinction in its section on defining and non-defining relative clauses.

Find the missing role inside the relative clause

Start with two sentences: The exhibition opens on Friday. I helped organise the exhibition. Join them:

The exhibition, which I helped organise, opens on Friday.

Which replaces the object of organise. Do not repeat that object: ~~which I helped organise it~~. Compare The exhibition, which opens on Friday, is free. Here which is the subject of opens. The pronoun remains necessary in both non-defining versions.

For possession, join I met Amira. Her sister runs the café:

I met Amira, whose sister runs the café.

Whose is followed by the thing possessed: whose sister, whose car, whose research. It can also refer to things: The company, whose offices are nearby, employs fifty people.

Which can comment on an event

The train was cancelled, which meant we had to take a bus uses which to refer to the cancellation as a whole. Compare We took a bus, which was crowded, where which refers to the bus.

If there is more than one possible reference, rewrite explicitly: The cancellation meant we had to take a bus. Clear reference matters more than fitting several facts into a single long sentence. Use non-defining clauses to add useful information, while keeping the main message easy to follow.