commas with adjective clauses
(A similar topic is discussed in French in the article virgule avec les relatives explicatives et déterminatives.)
Before deciding whether to use a comma with an adjective clause, you must determine whether the clause is essential or non-essential.
An essential clause (also known as a restrictive clause) is one that identifies or defines the word it modifies.
Do not use commas with an essential adjective clause:
- The sailing vessel that is engraved on the Canadian dime is the famous schooner Bluenose.
[The clause identifies by telling us which sailing vessel is meant.]
- Tommy Prince was a soldier who had extraordinary skill and courage.
[The clause defines by telling us what kind of soldier is meant.]
Tip: You don’t need to use commas around an adjective clause starting with that, since it will always be essential.
A non-essential clause is one that merely gives secondary information about the noun it modifies. In this case, the noun is already clearly identified, without the clause.
Do use commas with a non-essential adjective clause. If the non-essential clause is in the middle of the sentence, it takes a comma before and after it:
- The Bluenose, which is engraved on the Canadian dime, was a famous Nova Scotia schooner.
- Tommy Prince, who had extraordinary skill and courage, was descended from Manitoba’s legendary Chief Peguis.
If the non-essential clause comes at the end of the sentence, it has a comma before it:
- The most famous Nova Scotia schooner is the Bluenose, which is engraved on the Canadian dime.
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