1. Singular possessives: To form a possessive of a singular noun or pronoun, add -‘s, even when the original word ends with s (for example: Mr. Atkins’s car, the process’s steps, Bill Gates’s billions, the waitress’s tips, Congress’s foot-dragging).
This rule has some exceptions:
a) Personal pronoun possessives have no apostrophe—for example, ours, its, theirs, yours.
b) Biblical or Ancient names that end in s use only an apostrophe to signal the possessive—for example, Sophocles’ tragedies, Jesus’ sacrifice, Eratosthenes’ sphere, Socrates’ death.
2. Plural possessives: For most plural possessives, simply add an apostrophe. For irregular plural nouns that don’t end in s, add -‘s—for example children’s, women’s, phenomena’s.
3. Double possessives: A few style guides say that double possessives—i.e., constructions like a buddy of mine or a specialty of Sara’s—are redundant, with the of already indicating possession. These guides would have you write instead a buddy of me or a specialty of Sara.
However, very few respectable publications actually follow this rule. In fact, making the noun non-possessive may even lead to confusion—for example:
We cooked a favorite recipe of my mother-in-law.
4. Joint possessives. Joint possessives can be tricky. When the two possessors own the same thing, use one -‘s at the end of the noun phrase—for example:
Spot and Fido’s food bowl
When the possessors own separate things, use -‘s for each one:
William’s and Wallace’s poems
