Defining vs Non-Defining Relative Clauses: The Comma Rule

Learn the difference between defining and non-defining relative clauses, when to use commas, which relative pronouns to choose, and how to avoid common errors.

May 17, 2026 Updated on May 17, 2026 Confusing Words , Exam Grammar 6 min read

Two sentences can contain the same words but mean different things — all because of commas.

My brother who lives in Pune is a doctor. (I have more than one brother. This one lives in Pune.)
My brother, who lives in Pune, is a doctor. (I have one brother. He happens to live in Pune.)

The first sentence uses a defining clause — it identifies which brother. The second uses a non-defining clause — it adds extra information. The commas make all the difference.

For related reading, see Which vs That and Sentence Types.

A defining relative clause identifies which person or thing you mean. It is essential to the meaning. No commas.

A non-defining relative clause adds extra information about a person or thing already identified. It can be removed without changing the core meaning. Use commas.

Rule box: No commas for necessary information (defining). Commas for extra information (non-defining).

Feature	Defining	Non-defining
Purpose	Identifies the noun	Adds extra information
Commas	No commas	Commas required
Can be removed?	No — meaning changes	Yes — core meaning stays
Relative pronoun	who, which, that	who, which (not that)

The students who study daily improve faster.
The book that I bought is excellent.
People who exercise regularly stay healthier.

The clause identifies which students, which book, or which people. Without it, the sentence is too vague.

My father, who is 65, still runs every day.
The Taj Mahal, which is in Agra, is a UNESCO World Heritage Site.
Dr. Sharma, who taught us grammar, has retired.

The clause adds information. The sentence works without it:

My father still runs every day.
The Taj Mahal is a UNESCO World Heritage Site.

In defining clauses, that or who/which are both acceptable.

The book that I bought is good.
The book which I bought is good.

In non-defining clauses, use who or which — never that.

Wrong: The book, that I bought, is good.
Right: The book, which I bought, is good.

These can also introduce both types of clause.

Defining: The place where we met is closed now.
Non-defining: Pune, where I grew up, is a beautiful city.

Defining: The man whose car was stolen called the police.
Non-defining: Mr. Desai, whose son is a doctor, lives next door.

Is the information necessary to identify the noun? → Defining, no commas.
Is the information just extra detail? → Non-defining, use commas.
Can you remove the clause and still know who/what is meant? → Non-defining.
Choose the pronoun: who (people), which (things), that (defining only), whose (possession).
Check commas. Non-defining clauses need commas on both sides (or one side if at the end).

My brother who lives in Pune is a doctor.

No commas = defining = I have more than one brother. The clause identifies which one.

My brother, who lives in Pune, is a doctor.

Commas = non-defining = I have one brother. The clause adds extra information.

The book, that I bought, is good.

Non-defining clause cannot use that.

The book, which I bought, is good.

The students who passed the exam received certificates. (defining — which students?)
My classmates, who passed the exam, celebrated. (non-defining — all of them passed)
The phone that I bought last week is already slow. (defining)
My iPhone, which I bought last week, is already slow. (non-defining)
The city where I was born is very crowded. (defining)
Mumbai, where I was born, is very crowded. (non-defining)
The teacher who taught us maths was very strict. (defining — which teacher?)
Mrs. Kapoor, who taught us maths, was very strict. (non-defining)
The car that caused the accident fled. (defining)
The truck, which caused the accident, fled. (non-defining)

Wrong: The book, that I bought, is good.
Right: The book, which I bought, is good.

That cannot introduce non-defining clauses.

Wrong: My father who is 65 still runs.
Right: My father**,** who is 65**,** still runs.

Without commas, the sentence implies you have more than one father.

Wrong: The students**,** who study hard**,** will pass.
Right: The students who study hard will pass.

Commas make it non-defining, which changes the meaning.

Wrong: The book who I bought is good.
Right: The book which I bought is good.

Use who for people, which for things.

In defining clauses, the relative pronoun can be omitted when it is the object.

The book (that) I bought is good. (object — can omit)
The man who called you is my uncle. (subject — cannot omit)

Wrong	Right	Why
The book, that I bought, is good.	The book, which I bought, is good.	No that in non-defining.
My brother who lives in Pune is a doctor. (one brother)	My brother, who lives in Pune, is a doctor.	Non-defining needs commas.
The students, who study, will pass.	The students who study will pass.	Defining has no commas.
The book who I bought is good.	The book which I bought is good.	Who for people only.

Add commas where needed and choose the correct pronoun.

My sister who lives in Delhi is a teacher.
The car which I bought last month broke down.
The movie that we watched yesterday was boring.
Mr. Verma who is our principal is very kind.
Error spotting: The book, that I borrowed, is interesting.
Error spotting: My father, who is a doctor is very busy.
Error spotting: The phone who I bought is not working.
Fill in the blank: The man ___ called you is waiting. (who / which)
Rewrite correctly: My laptop that I bought last year still works well. (one laptop, extra info)
Choose: The students ___ passed will receive awards. (who / , who ,)

My sister, who lives in Delhi, is a teacher. (one sister, extra info)
The car, which I bought last month, broke down. (extra info)
The movie that we watched yesterday was boring. (defining — which movie?)
Mr. Verma, who is our principal, is very kind. (extra info)
The book, which I borrowed, is interesting.
My father, who is a doctor, is very busy.
The phone which I bought is not working.
who — subject pronoun for a person.
My laptop, which I bought last year, still works well.
who — defining clause, no commas.

Rule: Defining = no commas, identifies the noun. Non-defining = commas, adds extra info. Never use that in non-defining clauses.

Memory trick: Commas mean “by the way.” No commas mean “I need to tell you which one.”

Revise these:

The students who study daily improve. (defining)
My brother, who lives in Pune, is a doctor. (non-defining)
The book that I bought is good. (defining)
The book, which I bought, is good. (non-defining)

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