As a freelance editor, I’ve noticed writers sometimes struggle with possessives. Understandable. The rules have changed over the years, so what you may have learned in school may no longer apply, at least not in the publishing industry.
The general rule, according to The Chicago Manual of Style (16th edition), is that most singular nouns take an apostrophe and an s to show possessiveness. Plural nouns (with the exception of a few irregular plurals, like children, that don’t end in s) show possessiveness by adding an apostrophe only. This rule extends to proper nouns.
the horse’s mouth
a bass’s stripes
the Lincolns’ marriage
dinner at the Browns’ (at the Browns’ place)
Here’s where it gets tricky. When forming the possessive of words and names ending in an unpronounced “s”, Chicago has returned to an old rule, where the possessive would be formed in the usual way by adding an apostrophe and an s. This should make it easier for everyone, because the rules are more consistent.
Please note: Some publishers will still prefer the other way of showing possessive with words and names ending in an unpronounced “s”, by simply adding an apostrophe, but Chicago doesn’t recommend this practice. However, Chicago does acknowledge an exception to the general rule. When the singular form of a noun ending in s is the same as the plural, adding an apostrophe only forms the possessives of both.
Politics’ true meaning
The United States’ role in international law
The National Academy of Sciences’ new policy
I hope this helps to sort out some of the confusion.
Note: All examples were taken directly from The Chicago Manual of Style.