In a previous post, I wrote about Boss’s Day. Why did I write Boss’s Day and not Boss’ Day? Because according to the Associated Press Stylebook, for singular common nouns ending in s you express possession by adding ’s.
Examples: The actress’s voice, the hostess’s invitation
However, if the next word begins with an s, then just add the apostrophe without the s.
Examples: The actress’ smile, the boss’ seat
BUT… for singular proper nouns, use only an apostrophe. (Why do they have to make it so hard?)
Examples: Agnes’ book, my Guinness’ smooth flavor
Exception: St. James’s Palace (Don’t ask—this is why people hate grammar.)