Defining vs. Non-Defining Relative Clauses

What a Relative Clause Is

A relative clause is a part of a sentence that gives more information about a noun. It usually begins with a relative pronoun like who, which, that, or whose.

The woman who lives next door is a doctor.

This is the book that I told you about.

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

There are two types of relative clauses, and the difference matters for meaning and punctuation:

Type	What it does	Commas?
Defining	Identifies which one — essential	No commas
Non-defining	Adds extra info — not essential	Commas

Quick refresh: the relative pronouns

Before we get to the two types, here are the basic relative pronouns. (For a deeper look, see the Relative Pronouns lesson.)

Pronoun	Use it for	Example
`who`	people	`the man who called me`
`which`	things and animals	`the book which I bought`
`that`	people, things, animals (informal)	`the man that called me`
`whose`	possession (people, sometimes things)	`the boy whose bike was stolen`
`where`	places	`the city where I grew up`
`when`	times	`the day when we met`

Defining Relative Clauses

A defining relative clause tells us which one we mean. Without it, the sentence does not give enough information.

The woman who lives next door is a doctor.

If you remove the clause who lives next door, you get The woman is a doctor. — but which woman? The clause is essential. That is why it is called defining: it defines the noun.

Rules for defining clauses

Rule	Example
No commas	`The book that you lent me was great.`
Use `who`, `which`, `that`, `whose`	`the man who / that called`, `the book which / that I read`
`That` is fine here	`The film that we saw last night was funny.`
You can drop the pronoun if it is the object	`The book (that) I read was great.`

When you can leave the pronoun out

In a defining clause, if the pronoun is the object of the verb in the clause, you can usually drop it.

The man (who) I met yesterday was very kind.

The film (that) we saw was great.

The dress (which) she wore was beautiful.

But you cannot drop the pronoun when it is the subject of the clause.

The man who lives next door is a doctor. ✓

~~The man lives next door is a doctor.~~ ✗ (subject pronoun cannot be dropped)

A quick test: if there is already another subject inside the clause (I, she, we…), you can usually drop the relative pronoun.

Non-Defining Relative Clauses

A non-defining relative clause adds extra information about a noun that is already clear. If you remove the clause, the sentence still makes sense and identifies the same person or thing.

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

The main idea is My brother is visiting next week. The clause who lives in Berlin adds extra information. We already know which brother: it’s my brother.

Rules for non-defining clauses

Rule	Example
Always use commas	`Madrid, which is the capital of Spain, is beautiful.`
Use `who`, `which`, `whose` — never `that`	`Anna, who works with me, is from Italy.`
You cannot drop the pronoun	`My laptop, which I bought last year, is broken.`
Often used with proper nouns (names, unique things)	`Mount Everest, which is the highest mountain, is in Asia.`

The two big differences from defining clauses: commas and no that.

Comparing the Two

Feature	Defining	Non-defining
Commas	No	Yes
Can use `that`?	Yes	No
Can drop the pronoun (when object)?	Yes	No
Information	Essential — identifies the noun	Extra — already identified

Same sentence, different meanings

Look at how commas change the meaning:

My sister who lives in Paris is a chef. (I have more than one sister. The clause tells you which one — the one in Paris.)

My sister, who lives in Paris, is a chef. (I have one sister. The clause adds extra information about her.)

This is a real, important difference. Native readers rely on the commas to know whether the information is essential or extra.

Where, When, Whose

These pronouns also work in both types of clauses.

where (places)

The town where I grew up has a big market. (defining — which town?)

Florence, where my parents got married, is in Italy. (non-defining — extra info)

when (times)

I'll never forget the day when I met her. (defining)

Last Friday, when we had the storm, the train was cancelled. (non-defining)

whose (possession)

The boy whose bike was stolen is my neighbour. (defining)

My uncle, whose car I borrowed, lives in the next village. (non-defining)

Common Mistakes

Mistake	Better	Why
`The man, who called you, is my uncle.` (you have one uncle and the call is essential info)	`The man who called you is my uncle.`	If the clause is essential to identify the man, no commas.
`My brother who lives in Berlin is visiting.` (you have only one brother)	`My brother, who lives in Berlin, is visiting.`	Extra info about a unique person needs commas.
`Anna, that works with me, is from Italy.`	`Anna, who works with me, is from Italy.`	Do not use `that` in non-defining clauses.
`My laptop, I bought last year, is broken.`	`My laptop, which I bought last year, is broken.`	You cannot drop the pronoun in a non-defining clause.
`The man lives next door is a doctor.`	`The man who lives next door is a doctor.`	You cannot drop a subject relative pronoun.
`The book what I read was great.`	`The book that I read was great.`	Use `that` or `which`, not `what`, here.

Practice: Exercises

1 / 15

Choose the correctly punctuated sentence.

Summary

A defining relative clause tells you which one — it is essential. Use no commas, and you can use who, which, that, whose, where, or when. If the pronoun is the object, you can leave it out.

A non-defining relative clause adds extra information that is not essential. Use commas, never use that, and never drop the pronoun. These clauses are common around names, family members, and unique places.

The clearest test: read the sentence without the clause. If it still identifies the same person or thing clearly, the clause is non-defining (commas). If you no longer know which one, the clause is defining (no commas).