Relative Clauses (Defining & Non-Defining)

Here's a sentence you'd write without thinking: My cousin is visiting. Fine. But now you want to add that she lives in Cardiff. You could write a second sentence — She lives in Cardiff — or you could fold it straight into the first:

My cousin who lives in Cardiff is visiting.

That little add-on — who lives in Cardiff — is a relative clause. It hooks extra detail onto a noun so you don't have to keep starting new sentences. Once you can see the join, the choices stop feeling like guesswork.

What it is

A relative clause gives you more about a noun that's just been mentioned. It starts with one of these hook-words: who, which, that, whose. (The noun it hooks onto has a posh name — the antecedent — but you never have to use that word to get this right.)

the boy who scored the winner · the bus which was late · the film that we watched

Which hook-word?

The core three are easy once you know the split:

• who — for people: the girl who sits next to me.
• which — for things and animals: the phone which kept buzzing.
• that — for people or things, especially when you're saying which one you mean: the song that we learned.

Not sure? Turn the clause into a tiny sentence with he, she or it:

the coach who yelled → he yelled → a person doing something → who the bus which broke down → it broke down → a thing → which

You'll also meet whom now and then — it's the version for when the person is on the receiving end (the teacher whom I asked). Here's the honest bit: almost nobody uses it in everyday writing, and who is fine. If whom ever feels stiff, just drop the word altogether: the teacher I asked.

The big one: to comma or not to comma

This is the part that actually matters, so slow down. Some relative clauses are essential — you need them to know which person or thing is meant. Those get no commas:

Students who arrive late must sign in.

Not all students — only the late ones. The clause is doing real work, so it stays bare.

Other relative clauses are just extra — nice to know, but you'd still know who or what is meant without them. Those get commas on both sides:

My best friend, who's always late, missed the bus.

You've only got one best friend. The bit about being late is a bonus, so it sits in commas — like a little aside you could lift straight out.

The test: can you lift the clause out and still know who or what you mean?

• Yes → it's extra → use commas.
• No → it's essential → no commas.

Watch out

Getting the commas wrong doesn't just look untidy — it changes what you're saying:

• ✗ Students, who arrive late, must sign in. → this now claims all students arrive late.
• ✓ Students who arrive late must sign in. → only the latecomers. (That's what you meant.)

And the classic slip:

• ✗ The book who I borrowed… → ✓ The book which I borrowed… (a book is a thing → which)

One more, quickly: whose

whose shows that something belongs to the noun:

the boy whose phone rang · a team whose kit is bright orange

Don't be thrown when it turns up with a thing — the tree whose leaves fell is completely correct. English just doesn't have a smoother word for it, so whose does the job for people and things alike.

Quick check

1. Fill the gap: The scientist ___ discovered the vaccine visited our school.

2. Add commas if they're needed (you have only one brother): My brother who lives in Leeds is a nurse.

3. Which is right, and why? the museum which we visited or the museum who we visited?

4. True or false: whose can only be used about people.

Here or in the US?

Two small differences worth knowing. US writing is often stricter: it likes that for the essential (no-comma) clauses and keeps which for the extra, comma'd ones. Here in the UK, both that and which are fine in essential clauses — just be consistent. And whom is quietly fading on both sides of the Atlantic.

Where next

• How commas set off extra information — the same comma idea, beyond relative clauses.
• Who, whom, which, that — and whose — the spelling trap that looks like this but isn't (who's = who is).
• Joining sentences: and, but, so — other ways to fold two ideas into one.

Nobody's born knowing when a clause needs commas — but you've now got the one test that decides it every time: lift it out. If the sentence still knows what it means, add the commas. If it doesn't, leave them off. That single question does most of the work.