Commas

Q. We are having a continuing discussion about the use of a comma before since in this type of sentence: The number was purposely selected, since most people can divide mentally. My understanding is that if the subordinate clause follows the main clause, no comma precedes the conjunction; however, I saw the following sentence in the Q&A and am now confused. “Be aware, however, that the figures may depart from Chicago style in some details, since they are taken from actual manuscripts and published books or journals.”

A. The rule you remember is only half the rule. Please see CMOS 6.31: “A dependent clause that follows a main clause should not be preceded by a comma if it is restrictive, that is, essential to the meaning of the main clause. . . . If the dependent clause is merely supplementary or parenthetical, it should be preceded by a comma” (emphasis added). Please see 6.31 for examples.