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.

Essential clause

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.

Non-essential clause

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.