What is a non-defining relative clause?

It is a relative clause that adds extra information about a noun you have already identified. It is set off with commas, it cannot use that, and the relative pronoun cannot be dropped. Dr Elowen Hart, who restored the plaques, gave the talk still names the same person if you remove who restored the plaques.

Can I use that in a non-defining relative clause?

No. That is a defining-clause word. After a comma, use who for people and which for things: Ribblehead viaduct, which spans Batty Moss, was completed in 1875. If you want to keep that, remove the commas and accept the defining meaning: the viaduct that spans Batty Moss, said when more than one viaduct is in play.

Why can't I omit the relative pronoun?

Omission is a defining-clause pattern, used when the pronoun is an object (the plaques Elowen restored). A non-defining clause is punctuated as a separate unit and must keep who, which, whose, where, or when. The plaques, which Elowen restored in 2019, are back on the piers cannot lose which.

What is the difference between My sister, who lives in Truro and My sister who lives in Truro?

With commas, you have one sister, and who lives in Truro is extra. Without commas, you have more than one sister, and the clause picks out the one in Truro. The words are the same; the commas change the family.

Can which refer to a whole sentence?

Yes, in a non-defining clause at the end: The coble came in late, which meant we missed the last bus. Here which stands for the whole previous fact, not for a single noun. What cannot do this job. Keep the comma, and place the which-clause next to the idea it comments on.

English grammar · Intermediate

Non Defining Relative Clauses: Commas, Which, and No That

Written by Eleanor Whitfield, Lead English Curriculum Editor · Reviewed September 2026

A non-defining relative clause adds extra information about a noun that is already identified. It sits between commas (or after a comma at the end of the sentence), it cannot use that, and the relative pronoun cannot be left out. Removing the clause leaves a complete sentence whose meaning is still clear. Defining relative clauses, which identify which one you mean and take no commas, are a separate lesson.

Dr Elowen Hart, who restored the Ribblehead viaduct plaques, gave the talk.

commas round extra information about a person already identified

What it is

When the noun is already unique or already named, a relative clause is not there to pick it out. It is there to add a fact. Dr Elowen Hart is one person; who restored the Ribblehead viaduct plaques is extra. English marks that extra status with commas, and it refuses the tools that defining clauses use: that is not allowed, and the pronoun cannot be dropped. Take the clause away and the sentence still names the same person or thing. That is the test.

The harpsichord, which had been in the vestry since 1912, was tuned on Thursday. (We already know which harpsichord; the clause adds history.) The harpsichord that had been in the vestry since 1912 was tuned on Thursday. (Now the clause identifies which harpsichord: defining, no commas.)

How to spot it

Look for a comma before who, which, whose, where, or when after a noun, and another comma (or a full stop) when the clause ends. If you can delete the clause and still know exactly who or what is meant, it is non-defining.

Dr Elowen Hart, who restored the plaques, gave the talk. (the name already identifies her)
Ribblehead viaduct, which spans Batty Moss, was completed in 1875. (a unique place; which, not that)
Her brother, whose kiln is in Hebden, glazed the tiles. (whose stays; it cannot be omitted)
We reached Staithes, where the cobles come in at dusk. (where after a named place, with a comma)

That never starts a non-defining clause. The harpsichord, that had been in the vestry, is an error. Switch that to which, or drop the commas and make the clause defining.

Non-Defining Relative Clauses Quick Reference

Rules that apply only to non-defining clauses

Feature	Non-defining	Defining (for contrast)
Commas	Required around the clause	No commas
that	Never	Common for people and things
Omitted pronoun	Never: who, which, whose, where stay	Often possible when the pronoun is an object
who / which / whose / where / when	Yes	Yes
Noun already identified	Yes: a name, the only one, shared knowledge	No: the clause picks which one
Removing the clause	Sentence still complete and specific	Sentence becomes vague or changes meaning

Common Non-Defining Relative Clauses Examples in English

Non-defining clauses grouped by the extra information they add, each with the commas that mark it as extra:

Extra Facts about People

Dr Elowen Hart, who restored the Ribblehead plaques, gave the talk.: who as subject, name already unique
Iosif, whom the choir had elected as librarian, catalogued the parts.: whom as object, formal and kept
My aunt Sioned, whose coble still fishes out of Staithes, sent the crabs.: whose cannot be dropped
The soloist, who had travelled from Truro, arrived during the rehearsal.: the plus a unique role in context

A name, a unique role (the soloist, my aunt), or a noun the listener can already identify all license a non-defining clause.

Extra Facts about Things and Places

Ribblehead viaduct, which spans Batty Moss, was completed in 1875.: which for a unique structure
The harpsichord, which had been in the vestry since 1912, was tuned on Thursday.: which, commas, extra history
We walked to Staithes, where the cobles come in at dusk.: where after a named village
March, when the mill race thaws, is the noisiest month in the yard.: when after a named time

Which is the default for things. Where and when work after a named place or time. That is not available in any of these sentences.

Why the Pronoun Must Stay

The plaques, which Elowen restored in 2019, are back on the piers.: which as object: still required
Iosif, who the choir trusts with the archive, keeps the key.: who as object in everyday style, not omitted
The vestry, which we unlocked with a skeleton key, smelled of beeswax.: object which cannot be dropped after a comma

In a defining clause you could write the plaques Elowen restored. After a comma that drop is impossible: the plaques, Elowen restored, is not English.

How Non-Defining Relative Clauses Work

Commas, the ban on that, and a pronoun that cannot be dropped are one system, not three separate quirks.

Commas Mark the Clause as Extra

A pair of commas (or a comma and a full stop) tells the reader that the clause can be lifted out. In speech there is a small pause at each comma. Without the commas, the same words become a defining clause and the meaning changes: you are now picking which person or thing, not adding a fact about one you already named.

The harpsichord, which had been in the vestry, was tuned on Thursday.

commas: extra history about a known instrument

Commas = extra, not identifying.

That Is Never Used

Non-defining clauses take who for people, which for things, whose for possession, and where or when for places and times. That is a defining-clause word. Putting that after a comma is one of the most reliable tests of an error: switch to which or who, or remove the commas and accept the defining meaning.

Ribblehead viaduct, which spans Batty Moss, was completed in 1875.

which after a comma, never that

After a comma: who or which, not that.

The Relative Pronoun Cannot Be Omitted

Defining clauses often drop an object pronoun (the plaques Elowen restored). Non-defining clauses never do. Who, which, whom, whose, where, and when all stay, even when they are the object of the clause. The commas have already marked the clause as a separate unit; it needs its own subject or object on the page.

The plaques, which Elowen restored in 2019, are back on the piers.

which as object must remain

Never drop the pronoun after a comma.

The Noun Must Already Be Identified

Use a non-defining clause after a proper name, after a unique role in the context (the soloist, my father, this harpsichord), or after a noun both speakers can already pick out. If the listener still needs to know which one, you need a defining clause and no commas. A generic noun such as a plaque or people rarely takes a non-defining clause, because there is nothing unique to add to.

Dr Elowen Hart, who restored the plaques, gave the talk.

a name is already unique, so the clause is extra

Already identified: non-defining. Which one: defining.

Common Mistakes with Non-Defining Relative Clauses

Incorrect: Ribblehead viaduct, that spans Batty Moss, was completed in 1875.

Correct: Ribblehead viaduct, which spans Batty Moss, was completed in 1875.

That cannot open a non-defining clause. After a comma, things take which and people take who. Keep that for defining clauses with no commas.

Incorrect: The plaques, Elowen restored in 2019, are back on the piers.

Correct: The plaques, which Elowen restored in 2019, are back on the piers.

A non-defining clause cannot drop the relative pronoun. The commas have opened a separate unit, and that unit needs which (or who) on the page.

Incorrect: My sister who lives in Truro, plays the bassoon.

Correct: My sister, who lives in Truro, plays the bassoon.

A single comma is not enough, and the missing first comma also turns the clause defining. If you have one sister, both commas are required. If you have several, drop both commas and keep the defining meaning.

Incorrect: The coble came in late, what meant we missed the last bus.

Correct: The coble came in late, which meant we missed the last bus.

A comment on a whole previous clause takes which, not what. What introduces a clause that stands as a noun: What meant we missed the bus was the late coble.

Which Referring to a Whole Clause

Which can comment on the whole idea of the previous clause, not on a single noun. The comma is required, and that is still impossible.

A comment on the whole fact

After a complete clause, a comma plus which can mean and this fact. The which-clause then takes a verb that fits a situation, not a thing: which surprised us, which meant we walked, which was lucky.

The coble came in late, which meant we missed the last bus to Whitby.: which = the whole previous fact
Elowen finished the plaques ahead of the freeze, which was fortunate.: which comments on the finishing
Iosif mislaid the vestry key, which delayed the rehearsal by twenty minutes.: which = that mishap

What cannot do this job after a comma. The coble came in late, what meant we missed the bus is an error. What starts a clause of its own (What delayed us was the key).

Keep which next to the idea it comments on

Place the which-clause immediately after the fact it refers to, usually at the end of the sentence. If another noun sits between them, the reader may attach which to that noun by mistake.

The kiln cooled overnight, which saved the glaze.: which clearly refers to the cooling
She posted the scores to Truro, which solved the parts shortage.: which = posting the scores

If which could attach to a nearby noun, rewrite: Elowen finished the plaques, and that timing was fortunate, rather than a ambiguous which.

Quick Check: Non-Defining Relative Clauses

Add commas and the correct relative word, or mark the sentence as defining.

1. Dr Elowen Hart ___ restored the plaques gave the talk. (extra fact)

Answer: , who restored the plaques,

name is unique: non-defining who, two commas

2. Ribblehead viaduct ___ spans Batty Moss was completed in 1875.

Answer: , which spans Batty Moss,

which, not that, after a named place

3. The plaques ___ Elowen restored in 2019 are back on the piers. (extra fact, object pronoun)

Answer: , which Elowen restored in 2019,

object which cannot be omitted

4. The coble came in late, ___ meant we missed the last bus to Whitby.

Answer: which

which comments on the whole previous clause

5. The harpsichord ___ had been in the vestry since 1912 was the only one in the village. (identifying)

Answer: that (or which, no commas)

defining: no commas, that allowed

Non-Defining Relative Clauses FAQs

What is a non-defining relative clause?: It is a relative clause that adds extra information about a noun you have already identified. It is set off with commas, it cannot use that, and the relative pronoun cannot be dropped. Dr Elowen Hart, who restored the plaques, gave the talk still names the same person if you remove who restored the plaques.
Can I use that in a non-defining relative clause?: No. That is a defining-clause word. After a comma, use who for people and which for things: Ribblehead viaduct, which spans Batty Moss, was completed in 1875. If you want to keep that, remove the commas and accept the defining meaning: the viaduct that spans Batty Moss, said when more than one viaduct is in play.
Why can't I omit the relative pronoun?: Omission is a defining-clause pattern, used when the pronoun is an object (the plaques Elowen restored). A non-defining clause is punctuated as a separate unit and must keep who, which, whose, where, or when. The plaques, which Elowen restored in 2019, are back on the piers cannot lose which.
What is the difference between My sister, who lives in Truro and My sister who lives in Truro?: With commas, you have one sister, and who lives in Truro is extra. Without commas, you have more than one sister, and the clause picks out the one in Truro. The words are the same; the commas change the family.
Can which refer to a whole sentence?: Yes, in a non-defining clause at the end: The coble came in late, which meant we missed the last bus. Here which stands for the whole previous fact, not for a single noun. What cannot do this job. Keep the comma, and place the which-clause next to the idea it comments on.