Defining and Non-Defining Relative Clauses: How Commas Change Who and What You Mean

Compare these two sentences:

The students who revised regularly passed the test.
The students, who revised regularly, passed the test.

The words are almost identical. The meaning is not.

The first suggests a larger group of students, with the regularly revising students selected as a subgroup. The second presents the students as an already known group and adds that they revised regularly.

The commas are not decorative. They signal a different information structure.

The deeper question is: does the relative clause identify the noun, or does it merely add information about a noun whose identity is already clear?

What is a relative clause?

A relative clause gives more information about a noun or noun phrase. It is commonly introduced by:

who
which
that
whose
whom

Instead of:

I met a woman. The woman teaches Biology.

we can write:

I met a woman who teaches Biology.

Defining clauses identify

Take:

The teacher who teaches Chemistry is absent today.

If the school has several teachers, the clause who teaches Chemistry helps identify which teacher.

Remove it:

The teacher is absent today.

The reference may become unclear.

This is a defining relative clause.

Non-defining clauses add information

Now:

Mr Lim, who teaches Chemistry, is absent today.

The name Mr Lim already identifies the person.

The clause adds supplementary information.

Remove it:

Mr Lim is absent today.

The same person is still identified.

“Essential” does not mean “important”

Students are often told that defining clauses contain essential information.

Suppose:

My mother, who is recovering from surgery, will not attend the event.

The surgery may be extremely important. But it is not essential to identifying my mother.

In this grammar topic, “essential” means:

essential to identifying which person or thing is meant.

Defining clauses restrict the set

The books that were damaged were removed.

Imagine 100 books, of which 12 were damaged.

The clause that were damaged restricts the larger set to a subgroup.

This is why defining clauses are also called restrictive clauses.

Non-defining clauses do not select a subgroup

The books, which were damaged, were removed.

This presents the books as an already identified group and adds the information that they were damaged.

The sentence strongly suggests that all the relevant books were damaged.

One pair of commas can therefore change the size of the set described.

The brother example makes the difference clear

His brother who works at the supermarket is a friend of mine.

This implies he may have more than one brother.

His brother, who works at the supermarket, is a friend of mine.

This normally suggests one already identified brother. The clause merely adds where he works.

Meaning should come before punctuation

What noun does the clause describe?
Without the relative clause, do I still know exactly which person or thing is meant?
If no, the clause is likely defining.
If yes, it may be non-defining.
Then apply punctuation.

This turns commas into a consequence of meaning.

Relative pronouns carry grammatical roles

The student who won the prize smiled.

Here who is the subject of won.

The student who I met smiled.

Here who is the object of met.

The relative pronoun occupies a grammatical position inside the relative clause.

Object relative pronouns can sometimes disappear in defining clauses

They’re the people she met at Jon’s party.

The fuller version is:

They’re the people who/that she met at Jon’s party.

Because the relative pronoun is the object, English can often omit it in a defining clause.

But:

They’re the people who bought our house.

Here who is the subject and cannot simply disappear.

Non-defining clauses keep the relative pronoun

Clare, who I work with, is doing the marathon.

We do not normally write:

Clare, I work with, is doing the marathon.

That belongs to defining clauses, not non-defining clauses

Defining:

The book that I borrowed is missing.

Non-defining:

My book, which I borrowed yesterday, is missing.

This is why students should not learn who = people, which = things, that = either without a clause-type condition.

Which can refer to a whole previous clause

The train was cancelled, which caused serious delays.

Here which refers to the fact that the train was cancelled.

This is different from:

The train which was cancelled…

where which modifies the noun train.

Attachment matters

Consider:

I spoke to the manager of the branch who had called earlier.

Who had called?

World knowledge favours the manager, but long noun phrases can create genuine attachment ambiguity.

A useful editing rule is:

Place the relative clause close to the noun it is meant to describe.

Relative clauses are a compression tool

Without a relative clause:

The girl was standing beside the gate. She had found my wallet. She waved.

With one:

The girl who had found my wallet stood beside the gate and waved.

Identity and action are compressed into one sentence.

But long relative clauses can burden the reader

A clause carrying too many dates, causes and background details may be grammatically correct but hard to process.

Sentence complexity is useful only when the reader remains oriented.

Science uses defining relative clauses constantly

Cells that contain chloroplasts can carry out photosynthesis.

The clause defines a class of cells.

Now:

These guard cells, which contain chloroplasts, regulate the stomatal opening.

The noun phrase already identifies the cells; the relative clause adds information.

Mathematics uses them too

Values that satisfy the equation are solutions.

The clause defines the set.

The two solutions, which are both positive, can be checked by substitution.

The solutions are already identified.

History uses them for source identification

Sources that were written at the time of the event may provide direct contemporary evidence.

Defining: which sources?

The diary, which was written in 1942, describes food shortages.

Non-defining: the diary is already identified.

Relative clauses can accidentally overgeneralise

Students who use AI irresponsibly may weaken their learning.

This refers to a subgroup.

Students, who use AI irresponsibly, may weaken their learning.

This can suggest that all the relevant students use AI irresponsibly.

A comma can broaden a claim.

A practical scope model

large candidate set → defining clause → selected subset

versus:

already identified set → non-defining clause → supplementary property

This is why relative clauses connect naturally to quantifiers, specificity and reference.

Parent-friendly practice

My cousin who lives in Tampines is visiting.
My cousin, who lives in Tampines, is visiting.

Ask what each sentence suggests about how many cousins are relevant and whether the location is identifying or supplementary.

Teacher-friendly diagnosis

Reference error: wrong noun identified.
Defining/non-defining error: identifying information confused with supplementary detail.
Punctuation error: meaning correct but comma signalling wrong.
Relativiser error: who/which/that/whose chosen incorrectly.
Gap error: unnecessary pronoun added.
Agreement error: verb inside relative clause does not agree.
Attachment error: clause placed beside wrong noun.

Singapore relevance

Singapore Secondary English requires increasingly complex sentences while maintaining clarity, grammar and punctuation.

Relative clauses allow students to identify precise groups, add character detail, compress explanation and qualify claims.

They are also dangerous when punctuation changes scope accidentally.

Hilary Mantel as a high-level literary lens

Hilary Mantel is useful here only as a high-level lesson in perspective and selective detail. A descriptive clause can either identify which person matters or enrich a person already known.

The transferable lesson is: detail can either identify or enrich — and grammar distinguishes those jobs.

AI and relative clauses

AI-generated prose can insert commas around a relative clause because the sentence “sounds formal”, even when the commas change who is included.

A useful audit is:

For every relative clause, state whether it identifies a subset or adds supplementary information. Then check whether the commas and relative pronoun match that interpretation.

Connections eduKateAI can learn

noun reference → defining clause → selected subset

already identified noun → non-defining clause → supplementary information

comma pattern → information-status signal → scope interpretation

relative pronoun → grammatical role inside subordinate clause

AI edit → comma change → possible scope change

The public principle is that relative clauses control reference: they either narrow who or what is meant, or add information to an identity already secure.

Related eduKateSG resources

Research basis

This article draws on current Cambridge Grammar guidance and recent 2025–2026 research on EFL relative-clause use, filler-gap processing and academic writing.