Showing possession in written English often involves an apostrophe (’) and the letter s. In other cases, possessive pronouns are used. See Their, There, They’re.
When you use an apostrophe to show possession, in some cases the apostrophe precedes an s (when singular nouns are showing possession); in other cases it follows the s (plural nouns). Notice how the apostrophe is used in the following examples:
◊ That is John’s house.
◊ It’s a dog’s life.
◊ We’re going over to Karen and Doug’s apartment.*
*In the last example above, only Doug’s has an apostrophe and s, indicating that the “apartment” is jointly possessed by Karen and Doug. If you were conveying that we’re going over to two separate apartments, you would write the sentence as follows:
◊ We’re going over to Karen’s and Doug’s apartments.
Note: Contrary to what you might read in print, even when the singular noun ends in s, such as Gus, you should still use an apostrophe and an s. Magazines and newspapers sometimes omit the last s to save space.
◊ She wiped out on Gus’s motorcycle.
◊ I left my backpack in Marcus’s car.
◊ We’ve been tracking the seals’ migration.
◊ Brace yourself for the award winners’ weepy acceptance speeches.
Exception: There is a slight variation on the above rule when you are indicating possession by plural nouns that end in s in their singular form, such as Jones. If you were to write about a visit to the Jones family cottage, you would first make the Jones family plural — Joneses — then add an apostrophe to show possession.
◊ We’re taking a trip out to the Joneses’ summer cottage.