Headlines and Titles of Works

Q. Hello! I work as a proofreader in retail, and we often use “on sale” in headlines. I’m not sure if “on” is acting as a preposition or an adverb, therefore I’m not sure if it should be capitalized in a headline like this: “Now on Sale.” Thank you!

A. In “on sale,” on is a preposition and sale is its object. The expression itself is a phrasal adjective. Your headline is shorthand for “This Item Is Now on Sale” (or “These Items Are Now on Sale”)—in which the phrase “on Sale” modifies the understood item (or items). Chicago’s exceptions to its rules for headline-style capitalization do not extend to the use of prepositions in phrasal adjectives (except in common Latin phrases like “de facto”). So to follow Chicago style, write “Now on Sale.” See CMOS 8.159.