Non-Defining Relative Clauses

Two kinds of relative clause

Every relative clause attaches extra information to a noun. The crucial question is whether that information is essential or not.

The student who passed the exam got a scholarship. (defining — tells you which student)

Maria, who passed the exam, got a scholarship. (non-defining — Maria is already identified; the clause just adds extra detail)

The second sentence still works if you remove the relative clause: Maria got a scholarship. That removability test is the heart of this lesson.

Clause type	Function	Punctuation	Removable?
Defining	Identifies which one	No commas	No — meaning changes
Non-defining	Adds extra information	Commas required	Yes — main clause still works

This lesson focuses entirely on the non-defining type — the one with commas, the one that drops that, and the one that lets which refer to a whole idea.

The Comma Rule (Mandatory)

Non-defining clauses are always marked with commas. Both sides — opening and closing — are required.

My brother, who lives in Tokyo, is visiting next week.

The Eiffel Tower, which was built in 1889, attracts millions of tourists.

Our manager, whose office is on the third floor, is on holiday.

If the non-defining clause comes at the end of the sentence, you only need one comma (in front of it):

I called my sister, who was already at the airport.

We climbed Mount Fuji, which took us nine hours.

Test it: read the sentence aloud. Non-defining clauses sound like a parenthetical aside — your voice slightly drops and there’s a tiny pause. That pause is the comma.

Why commas matter for meaning

The presence or absence of commas can change the meaning entirely.

Sentence	Meaning
`My sister who lives in Berlin is a doctor.` (no commas)	I have several sisters; the one in Berlin is a doctor.
`My sister, who lives in Berlin, is a doctor.` (commas)	I have one sister; she happens to live in Berlin.

The commas tell the reader: this clause is not narrowing down which sister; it’s just extra information about the only sister I have.

`that` Is Forbidden in Non-Defining Clauses

that is fine in defining clauses (the book that I read), but it cannot introduce a non-defining clause. You must use who, whom, which, or whose.

Wrong	Correct
`My father, that is a teacher, retired last year.`	`My father, who is a teacher, retired last year.`
`London, that is the capital, has eight million people.`	`London, which is the capital, has eight million people.`
`My laptop, that I bought in 2020, still works fine.`	`My laptop, which I bought in 2020, still works fine.`

Likewise, you cannot omit the relative pronoun in a non-defining clause, even when it’s the object of the verb:

The painting, which I bought last year, is worth a fortune. Not: The painting, I bought last year, is worth a fortune.

In defining clauses, you can drop the pronoun (The painting [that] I bought last year). In non-defining clauses, you can never drop it.

Choosing the Pronoun

Pronoun	Used for	Example
`who`	people (subject or object)	`My uncle, who runs a bakery, sent us bread.`
`whom`	people (object — formal)	`The director, whom I met last year, is retiring.`
`which`	things, animals, ideas	`The hotel, which has 200 rooms, is fully booked.`
`whose`	possession (people or things)	`Anna, whose son is a pilot, lives nearby.`
`where`	places	`Edinburgh, where my parents grew up, is beautiful.`
`when`	times	`The 1990s, when I was a student, were good years.`

Use whose for things as well as people:

The novel, whose ending shocked everyone, won the prize.

Formal Patterns: `of which`, `of whom`, `in which`, etc.

In formal writing, prepositions in non-defining clauses move in front of the relative pronoun. The pronoun then becomes whom (for people) or which (for things) — never who or that.

Informal vs formal

Informal	Formal
`The book, which I told you about, is sold out.`	`The book, about which I told you, is sold out.`
`My colleagues, who I work with, are excellent.`	`My colleagues, with whom I work, are excellent.`
`The house, which we lived in for ten years, has been demolished.`	`The house, in which we lived for ten years, has been demolished.`

The informal versions strand the preposition at the end of the clause. The formal versions move it forward and force whom (not who).

Quantity expressions: `some of which`, `most of whom`

A particularly useful formal pattern combines a quantifier with of which / of whom. It lets you describe what fraction of a group has some property.

The company has 300 employees, **most of whom** are based in Europe.

She owns six houses, **two of which** she rents out.

The students, **none of whom** had studied, failed the exam.

There were 50 candidates, **all of whom** were highly qualified.

Quantifier	+ of which (things)	+ of whom (people)
`all`	`all of which`	`all of whom`
`most`	`most of which`	`most of whom`
`some`	`some of which`	`some of whom`
`several`	`several of which`	`several of whom`
`none`	`none of which`	`none of whom`
`both`	`both of which`	`both of whom`
`neither`	`neither of which`	`neither of whom`
`a few / many / few`	`a few of which`	`a few of whom`
`numbers`	`two of which`, `ten of which`	`two of whom`, `ten of whom`

These structures elevate a sentence — they’re common in academic, business, and journalistic writing.

`which` Referring to a Whole Clause

This is one of the most useful features of non-defining clauses. which can refer to the entire previous clause, not just a single noun.

He arrived two hours late, which annoyed everyone. → His arriving two hours late annoyed everyone — which refers to the whole event, not to a noun.

She sang beautifully, which surprised us all.

The trains were cancelled, which meant we had to drive.

He has decided to retire, which is a real loss for the team.

I forgot her birthday, which I still feel guilty about.

This pattern always uses which (never that or what), is always preceded by a comma, and always describes a fact, event, or situation as a whole.

Don’t use what for this: He arrived late, what annoyed everyone. is wrong. What doesn’t introduce a relative clause; it introduces a noun clause (see the noun-clauses lesson).

Common Mistakes

Mistake	Better	Why
`My father, that is 70, still plays tennis.`	`My father, who is 70, still plays tennis.`	`that` is forbidden in non-defining clauses; use `who` for people.
`Paris which is the capital of France is beautiful.`	`Paris, which is the capital of France, is beautiful.`	Non-defining clauses need commas on both sides.
`My brother who lives in Madrid is a chef. He's the only brother I have.`	`My brother, who lives in Madrid, is a chef.`	Without commas, the clause becomes defining and implies you have other brothers.
`She has three cars, two of them are electric.`	`She has three cars, two of which are electric.`	Use `of which` (not `of them`) to join a non-defining clause to its main clause.
`The students, who I teach them every Monday, are excellent.`	`The students, who I teach every Monday, are excellent.`	Don’t repeat the object pronoun — `who/whom` already serves that role.
`He missed the train, what made him late.`	`He missed the train, which made him late.`	Use `which` (not `what`) for a clause-referring relative.
`My laptop, I bought last year, is broken.`	`My laptop, which I bought last year, is broken.`	You cannot drop the relative pronoun in a non-defining clause.
`The book about that I told you is sold out.`	`The book about which I told you is sold out.` (formal)	Prepositions never sit before `that`; use `which` instead.

Quick-Reference Cheat Sheet

You want to…	Use…	Example
Add extra info about a person	`, who`	`Lara, who lives in Spain, called me.`
Add extra info about a thing	`, which`	`My phone, which is new, broke.`
Show possession	`, whose`	`Tom, whose dog is enormous, walks past every day.`
Refer to a whole clause	`, which`	`She lied, which upset me.`
Describe part of a group	`, … of whom / of which`	`He has five sisters, two of whom are doctors.`
Use formally with a preposition	`prep + whom / which`	`The team, with whom I worked, won.`

Practice: Exercises

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Which sentence is a non-defining relative clause?

Summary

Non-defining relative clauses add extra, removable information to a noun (or, in the case of , which …, to a whole clause).

Commas are mandatory — both sides if the clause is in the middle, one comma if it’s at the end.
that is forbidden — use who (people), which (things), whose (possession), where (places), when (times).
You can’t drop the pronoun even when it’s the object — The car, I bought in 2020, is fast. is wrong.
Formal: preposition moves before the pronoun, and who becomes whom. The team with whom I worked.
some/most/all/none/two of whom/which is a powerful pattern for splitting a quantity out of a group.
, which can refer to the whole previous clause: He lied, which upset everyone. — never use what here.

The mental test never changes: if removing the clause leaves the sentence intact and clearly identifying its noun, you have a non-defining clause — punctuate it with commas, drop that, keep the pronoun.