English — Defining vs non-defining relative clauses
A defining relative clause tells you which person or thing is meant and cannot be removed: "The book that I lent you is overdue." A non-defining clause just adds extra information and is set off by commas: "This book, which I read last year, is overdue."
How it works
A defining clause takes no commas, can use that in place of who/which, and often drops the pronoun when it is the object: "The man who/that lives here" but "The man (that) I met." A non-defining clause needs commas, cannot use that, cannot drop the pronoun, and keeps who or which: "My uncle, who lives in Rome, called."
Compared side by side
| Feature | Defining | Non-defining |
|---|---|---|
| Commas | None | Required |
| Subject example | The woman who/that called is my boss. | Anna, who called earlier, is my boss. |
| Object example | The car (that) we bought is red. | That car, which we bought last week, is red. |
| Can use "that"? | Yes | No |
| Drop the pronoun? | Yes, when object | Never |
| Meaning added | Identifies (essential) | Adds extra info |
Good to know
The commas change the meaning, so choose carefully. "My sister who lives in Paris is a doctor" (defining) implies you have more than one sister; "My sister, who lives in Paris, is a doctor" (non-defining) implies just one sister and adds where she lives. Never use that after a comma: write "Rome, which is old", not "Rome, that is old."