Non-defining Adjective Clauses - Rules

Here are three rules to follow when using non-defining adjective clauses.

If you follow these simple rules, you will avoid mistakes when you use this type of clause.

1. Always use commas to separate a non-defining adjective clause from the rest of the sentence.

Do this	Ottawa, which is the capital of Canada, is situated on the border between Ontario and Quebec.	Explanation: Do you see the commas? Non-defining clauses need commas.
Don't do this	Ottawa which is the capital of Canada is situated on the border between Ontario and Quebec.	Explanation: This one doesn't have the necessary commas.

2. Relative pronouns: never use "that" as a relative pronoun in a non-defining adjective clause.

Do this	Ottawa, which is the capital of Canada, is perhaps the fourth largest city in the country.	Explanation: The relative pronoun, "which", is correct.
Don't do this	Ottawa, that is the capital of Canada, is perhaps...	Explanation: The relative pronoun, "that", is wrong.

3. Relative pronouns: you can never omit the relative pronoun (unlike defining adjective clauses)

Do this	I went to their concert, which gave me a good idea of what kind of music they play.	Explanation: This one has a relative pronoun, which is good.
Don't do this	I went to their concert, gave me a good idea of what kind of music they play.	Explanation: This one omits the relative pronoun, which is wrong.

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