Double, Double Toil and Trouble

Double possessives (when something is possessed by more than one person) can be doubly vexing if you’re unaware of the rules. Just remember two simple things: Simple thing no. 1: When there’s more than one owner – Gilbert and Sullivan possess something in common – consider them a single unit with ’s after the last possessor’s name. Gilbert and Sullivan’s operetta was a hit. Simple thing no. 2: If both Gilbert and Sullivan have their own hit operettas, each gets his own ’s. Gilbert’s and Sullivan’s operettas are hits.

Note: Double possessives also include constructions like “a paramour of my sister’s” – double because both “of” and the 's indicate possession. So you’d think that it would be wrong since only one thing is possessed. While it might be technically wrong (better, perhaps, to write “a paramour of my sister"?), it is how we speak and write naturally. It also can clear up confusion found in a phrase like “a picture of my brother” – is it a picture of my brother or a picture belonging to him? The double possessive makes it clear.