Q. In the first example in CMOS 8.161, the word than is capitalized: “Mnemonics That Work Are Better Than Rules That Do Not.” This does not seem to be in keeping with your general principles of title case. We’d be grateful if you would clarify.
A. Than is a conjunction in that title and therefore capped (see CMOS 8.160, rule 1; but see rule 4 for some conjunctions that are not capped). When than is a preposition in a title (“Younger than Springtime”), Chicago style lowercases it. (Check the examples in a good dictionary to figure out which part of speech a word is.)