Non-defining relative clauses

Some relative clauses tell you which person or thing is meant. Others just add an extra detail about something you've already identified — a side comment that could be lifted out of the sentence without losing the main point. That second kind is a non-defining relative clause, and it comes with its own small set of rules.

My brother, who lives in Berlin, is visiting next week. There's an obvious assumption behind that sentence: you already know which brother is meant — probably because you only have one. The clause who lives in Berlin just adds a detail; take it away, and My brother is visiting next week still says exactly who is visiting.

The quick version: a non-defining relative clause is set off by commas and adds extra, non-essential information. Use who / which / whose / whom — never that — and never drop the pronoun.

This article builds on defining relative clauses, which covers who, which, that, whose, and whom for clauses that narrow down which person or thing you mean. The basic choices for who, which, whose, and whom carry over here, but that does not. What's different is the punctuation, the impossibility of dropping the relative pronoun, and one new use of which.

Commas make the difference

Compare these two sentences:

• The students who finished early could leave. — no commas: only some students, the ones who finished early. Defining.
• Anna, who finished early, went home. — commas: extra detail about someone already identified by name. Non-defining.

The words in the clause can be identical — what changes is whether commas surround it, and what that implies about the noun before it. A non-defining clause is set off by:

• one comma, if the clause ends the sentence: I called my sister, who lives in Rome.
• two commas, if the clause sits in the middle: My sister, who lives in Rome, called me yesterday.

Read the sentence aloud: a non-defining clause is the part you'd say with a slight pause on either side, almost as an aside — drop it, and the sentence's core meaning survives untouched.

No that, and no dropping the pronoun

Two habits from defining clauses don't carry over here:

• ❌ My brother, that lives in Berlin, is visiting. → ✅ My brother, who lives in Berlin, is visiting. That is never used in a non-defining relative clause.
• ❌ My brother, lives in Berlin, is visiting. → ✅ My brother, who lives in Berlin, is visiting. The pronoun can never be dropped, even where it's the subject of the clause.
• ❌ My sister, I met in Rome, is a photographer. → ✅ My sister, who I met in Rome, is a photographer. The pronoun stays even when it's the object.

That last point is a real contrast with defining clauses, where an object pronoun is often dropped (the film I recommended). In standard English, a non-defining relative clause always needs an explicit relative pronoun — dropping it isn't optional, whether the pronoun is the subject or the object of its clause.

Whose and whom work the same way

Whose (possession) and whom (formal object pronoun for people) behave exactly as in defining clauses — just with commas added:

• The Garcías, whose daughter is a violinist, just moved in next door.
• Dr. Patel, whom I met at the conference, gave a fascinating talk.

Everyday English often prefers who over whom even here. But in a non-defining relative clause, that is still not possible.

Which can refer to a whole clause

A non-defining which can point back to an entire idea, not just a single noun — a genuinely new trick at this level:

• He arrived late, which annoyed his boss. (which = the fact that he arrived late)
• She passed the exam, which surprised everyone. (which = the fact that she passed)
• They forgot to invite him, which was a bit awkward. (which = the fact that they forgot)

This only works with which, always in a non-defining clause with a comma before it. Use it when which is meant to sum up the whole preceding situation, rather than pointing at one particular noun in the sentence.

Common mistakes

• ❌ My car, that broke down yesterday, is at the garage. → ✅ My car, which broke down yesterday, is at the garage. (never that in a non-defining clause)
• ❌ Her father, works as an engineer, is visiting this weekend. → ✅ Her father, who works as an engineer, is visiting this weekend. (the pronoun can't be dropped)
• ❌ My neighbour who is a doctor helped me. → ✅ My neighbour, who is a doctor, helped me. (if it's already clear from context which neighbour you mean, missing commas turn a non-defining aside into a defining clause that wrongly implies you're picking one out of several)
• ❌ We missed the train, that was really frustrating. → ✅ We missed the train, which was really frustrating. (referring back to a whole clause needs which, never that)
• ❌ I visited Paris which is the capital of France. → ✅ I visited Paris, which is the capital of France. (a non-defining clause always needs its comma, even when short)

Quick check

Decide whether each clause needs commas (non-defining) or not (defining), and choose the right pronoun.

1. My mother ____ taught me to cook is visiting this weekend. (she is my only mother)
2. The book ____ I borrowed from you was excellent.
3. He lost his passport, ____ ruined the whole trip.
4. Mr. Chen, ____ office is next to mine, is retiring soon.

Key takeaways

• A non-defining relative clause adds extra, non-essential information — removing it doesn't change who or what the sentence is about.
• It's set off by commas: one if it ends the sentence, two if it sits in the middle.
• That is never used in a non-defining relative clause — use who, which, whose, or whom instead.
• The pronoun can never be dropped in a non-defining clause, even when it's the object.
• Which can refer back to an entire preceding clause, not just a noun: He was late, which annoyed everyone.
• Missing commas can quietly turn a non-defining aside into a defining clause with a different meaning — don't skip them.