What are non-defining relative clauses?
A non-defining relative clause gives extra information about a noun. The noun is already specific and identified, so the clause adds details rather than defines it. Non-defining clauses are always separated by commas. For example: 'My sister, who lives in Paris, is a doctor.' We already know which sister—she is identified by being 'my sister'—so the clause 'who lives in Paris' adds extra information but doesn't define which sister we mean.
Non-defining vs. defining clauses
This is the key difference: defining clauses have NO commas and tell us which person or thing we are talking about. Non-defining clauses HAVE commas and give optional extra information. Compare: 'The student who finished first won a prize' (defining—tells us which student) and 'Tom, who finished first, won a prize' (non-defining—Tom is already identified, and 'who finished first' is extra information).
Relative pronouns in non-defining clauses
Use who for people, which for things and animals, and whose for possession. Important: in non-defining clauses, you cannot use 'that' as a relative pronoun. You must use who, which, or whose. You can also use where for places and when for times. Example: 'The café, which opened last year, is very popular' (correct) not 'The café, that opened last year, is very popular' (incorrect).
Defining vs. Non-Defining Relative Clauses: Side-by-Side Comparison
| Feature | Defining Relative Clauses | Non-Defining Relative Clauses |
|---|---|---|
| Form | Relative clause placed directly after the noun it modifies, without commas separating it from the rest of the sentence. | Relative clause placed after the noun it modifies, separated by commas (or dashes/parentheses in informal writing). |
| Comma Usage | No commas used. The clause is integral to the sentence structure and flows without a pause. | Commas required before and after the clause (unless the clause ends the sentence, in which case only a comma before it is needed). |
| Pronoun Choices | who, whom, which, that, whose, when, where — notably, that is commonly used and the relative pronoun can be omitted when it is the object of the clause. | who, whom, which, whose, when, where — that cannot be used, and the relative pronoun cannot be omitted. |
| Removability of the Clause | Cannot be removed without losing essential meaning. The clause identifies which person or thing is being discussed, making it grammatically and semantically necessary. | Can be removed without changing the core meaning of the sentence. The clause adds extra, supplementary information about a noun already clearly identified. |
| Effect on Meaning | Restricts or defines the noun — it narrows down which specific person, thing, or place is meant out of a larger group. Without the clause, the sentence becomes ambiguous or incomplete. | Adds extra detail — it provides additional, non-essential information about a noun already uniquely identified. Removing the clause leaves the main meaning intact. |
| Positive Example | The student who studies every night always passes her exams. (Identifies a specific student among many.) |
Maria, who studies every night, always passes her exams. (Maria is already identified; the clause simply adds information.) |
| Negative Example | The car that he bought last year doesn't start in cold weather. (Specifies which car out of potentially several.) |
His old Volkswagen, which he bought last year, doesn't start in cold weather. (The car is already uniquely identified by name.) |
| Question Example | Is the restaurant that opened last month still busy? (Defines which restaurant is being asked about.) |
Is Bella Italia, which opened last month, still busy? (The restaurant is already named; the clause provides background.) |
| Key Signal Words / Markers | that, who, which, whose, where, when (no commas); pronoun often omittable; noun is usually non-specific (e.g., a man, the people, the book). | who, which, whose, where, when (always with commas); noun is usually a proper noun or already uniquely identified (e.g., London, my brother, the President). |
| 🔑 Key Difference: A defining relative clause tells us which specific person or thing is meant — without it, we don't know who or what is being discussed. A non-defining relative clause adds bonus information about someone or something already clearly identified — it is always separated by commas, can never use that, cannot drop the relative pronoun, and can be removed from the sentence without changing its essential meaning. | ||
Formula
✔ Positive
Main noun (already identified)
+
,
+
relative pronoun (who/which/whose/where/when)
+
additional information
+
,
My sister, who is a doctor, lives in Paris.
Examples
My best friend, who I've known since childhood, is getting married next month.
Non-defining relative clause · Person · Extra information
The London Museum, which is free to enter, has excellent collections.
Non-defining relative clause · Thing · Extra information
Sarah's husband, whose family is Italian, speaks three languages.
Non-defining relative clause · Possession · Extra information
We visited Barcelona in 2019, when the weather was beautiful.
Non-defining relative clause · Time · Extra information
The restaurant, where we had dinner last week, has closed down.
Non-defining relative clause · Place · Extra information
Our English teacher, who studied at Oxford, makes grammar fun.
Non-defining relative clause · Person · Extra information
When to use it
Talking about people
Use non-defining relative clauses when describing a specific person and adding background information about them.
My colleague, who graduated from Cambridge, is leading the project.
Describing places and objects
Add extra details about a specific place or thing that is already clearly identified.
The local library, which was built in 1920, is being renovated next year.
Adding time context
Provide additional information about when something happened using 'when' in a non-defining clause.
We visited Rome in 2018, when they were hosting a special exhibition.
Professional writing
Use in formal contexts to provide additional context without making the sentence too complex.
Dr. Smith, whose research has been published widely, will give the keynote speech.
Signal words
who
which
whose
where
when
comma (,)
already identified
extra information
optional details
Common Mistakes
✕
Wrong
My father, that is a mechanic, works hard every day.
✓
Correct
My father, who is a mechanic, works hard every day.
Cannot use 'that' in non-defining clauses. Use 'who' for people instead.
✕
Wrong
The hotel, which we stayed in is very expensive.
✓
Correct
The hotel, which we stayed in, is very expensive.
Non-defining clauses must have commas on both sides (or at least at the beginning if at the end).
✕
Wrong
John who is my cousin arrived yesterday.
✓
Correct
John, who is my cousin, arrived yesterday.
Missing commas. 'John' is a proper noun already identified, so the clause is non-defining and needs commas.
✕
Wrong
The park where we played, is near my house.
✓
Correct
The park where we played is near my house.
Comma placement is wrong. If the relative clause is defining (identifies which park), no commas are needed.
KEY TAKEAWAYS
What to Remember
- Non-defining relative clauses add extra information about a noun that is already identified.
- Always separate non-defining clauses with commas before and after the clause.
- The noun in a non-defining clause is specific and already known to the reader.
- Defining clauses identify which noun you mean, while non-defining clauses just add details.
- Common mistake: don't use commas with defining clauses, only with non-defining clauses.