Q. Is there a rule that I’ve missed somewhere that says there should always be a comma before the word “then” if “then” is at the end of a sentence? For example: It’s settled, then. Sometimes it sounds fine; other times it seems more like an obstacle to the flow of the sentence. But a rule is a rule, so if you can point me to the correct section in CMOS, I’ll stop turning up my nose at this construction.

A. It’s dangerous to make a rule saying that you always have to put a comma in front of a particular word, so we avoid doing that. The trick is to determine whether a comma is needed. In the case of “then” it’s rarely needed when the word means “at that time”; it’s often needed when it means “in that case.” The comma shows the meaning:

Meet me at the hot tub then. (Then = at the appointed time.)

Meet me at the hot tub, then. (Then = so, it’s decided.)