A defining clause says which person or thing you mean. A non-defining clause adds detail about one already identified. Commas show the reader which kind you are writing.
This lesson is part of grammar choices that change meaning. It builds on the clause work in building accurate complex sentences.
How do you tell the two kinds apart?
Use the remove-the-clause test. Delete the clause and ask whether the sentence still identifies the same person or thing.
If the sentence becomes vague, the clause is defining and takes no commas. If the sentence still points to the same noun, the clause is extra and takes a comma on each side.
Worked example: same words, different meaning
Read these two original sentences.
A. “The students who completed the survey received a certificate.”
B. “The students, who completed the survey, received a certificate.”
In A, the clause narrows the group. Only the students who completed the survey got a certificate. Delete the clause and the sentence says all the students got one, which changes the meaning, so the clause is defining.
In B, the commas make the clause extra information. All the students completed the survey and all received a certificate. Delete the clause and the group is still the same students.
A reader of B would believe nobody skipped the survey. If the writer meant only some students, the commas are a mistake.
The mistake: commas by feel
Students add commas wherever they pause when reading aloud. A pause is not the rule, so a defining clause ends up looking like extra information.
| Sentence | Commas | Meaning |
|---|---|---|
| The book that I borrowed was long. | none | identifies which book |
| The book, which I borrowed, was long. | yes | the book is already known |
| My brother who lives in Ipoh is a nurse. | none | I have more than one brother |
| My brother, who lives in Ipoh, is a nurse. | yes | I have one brother |
The last two rows show how commas change a fact about the writer’s family.
Which word goes with which clause?
For people, use “who” in both kinds. For things, use “that” or “which” in a defining clause, and only “which” with commas.
Use “whose” for possession in both: “The girl whose bag was lost reported it.” Check each clause with the test before you add or remove commas.
Check yourself
Decide whether the clause defines or adds, then punctuate. (1) “The teacher who marked my essay was Mr Hafiz, and he is new.” (2) “Penang which is famous for its food attracts visitors.” (3) “The shoes that I bought yesterday are too tight.”
Answer
(1) No commas in the clause. The clause identifies which teacher, so only the comma before “and” is needed.
(2) “Penang, which is famous for its food, attracts visitors.” Penang is already identified, so the clause adds detail and takes commas.
(3) “The shoes that I bought yesterday are too tight.” The clause identifies which shoes, so no commas.
What to study next
Finish the cluster with correcting a conditional while preserving whether the situation is real. Check punctuation with the English writing task checklist.
For a teacher to go through your clauses with you, see online one-to-one English tuition.