Headlines and Titles of Works
Q. In the first example in CMOS 8.160, 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 headline-style capitalization. We’d be grateful if you would clarify.
A. Than is a conjunction in that title and therefore capped (see CMOS 8.159, point 1; but see point 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.)