Adjective and adverb clauses. An adjective clause that begins with that is always restrictive; do not set it off with commas. An adjective clause beginning with which may be either restrictive or nonrestrictive; however, some writers prefer to use which only for nonrestrictive clauses, which they set off with commas.

RESTRICTIVE CLAUSES

The claim that men like seriously to battle one another to some sort of finish is a myth.

—John McMurtry, “Kill ’Em! Crush ’Em! Eat ’Em Raw!”

The adjective clause is necessary to the meaning because it explains which claim is a myth; therefore, the clause is not set off with commas.

The adjective clause is necessary to the meaning because it identifies the man, so it takes no commas.

NONRESTRICTIVE CLAUSES

I borrowed books from the rental library of Shakespeare and Company, which was the library and bookstore of Sylvia Beach at 12 rue de l’Odeon.

—Ernest Hemingway, A Moveable Feast

The adjective clause is not necessary to the meaning of the independent clause and therefore is set off with a comma.

An adverb clause that follows a main clause does not usually require a comma to set it off unless the adverb clause expresses contrast.

The adverb clause expresses contrast; therefore, it is set off with a comma.