Q. Hi there. I have a question regarding the use of double prepositions. Is there a rule against it? I tried to check for rules in CMOS, but I didn’t see any. I also checked a dictionary, and it says that “off of” is an idiom and is therefore correct.

A. That “off of” is an idiom does not mean it’s correct. In fact, it means that caution is required: many idioms are considered slang or informal. CMOS guidelines apply to formal speech and writing, and CMOS says never to use “off of” (see 5.220, under “off”). There is no rule against double prepositions, however. “I ran out of the house” and “He peered from behind the tree” are perfectly grammatical and idiomatic.