How to Form Plural Possessives in English
The plurals of most English nouns already end with the letter s. To show ownership, all you do is add an apostrophe after the s. Many people don’t believe it, but it is true. Take a look at these examples:
ten gerbils’ tiny teeth (the tiny teeth belong to ten gerbils)
many dinosaurs’ petrified teeth (the petrified teeth belong to a herd of dinosaurs)
a thousand sword swallowers’ sliced teeth (the sliced teeth belong to a thousand sword swallowers)
The of test works for plurals, too. If you can rephrase the expression using the word of, you may need an apostrophe. Remember to add the apostrophe after the letter s.
three days’ dental work on those false teeth = three days of dental work
sixteen years’ neglect on the part of Lulu’s dentist = sixteen years of neglect
two centuries’ pain of rotten teeth = two centuries of pain
Irregular plural possessives
The word teeth is plural, but teeth doesn’t end with the letter s. In other words, teeth is an irregular plural. To show ownership for an irregular plural, add an apostrophe and then the letter s (teeth’s). Check out these examples:
teeth’s cavities (The cavities belong to the teeth.)
children’s erupting teeth (The erupting teeth belong to the children.)
the three blind mice’s imaginary teeth (The imaginary teeth belong to the three blind mice.)
the women’s lipstick-stained teeth (The lipstick-stained teeth belong to the women.)
the mice’s cheesy teeth (The cheesy teeth belong to the mice.)
geese’s missing teeth (No teeth belong to the geese because, as of course you know, birds have beaks instead.)
Compound plural possessives
What happens when two single people own something? In real life they go to court and fight it out. In grammar, they (or you) add one or two apostrophes, depending upon the type of ownership. If two people own something together, as a couple, use only one apostrophe.
George and Martha Washington’s home (The home belongs to the two of them.)
Larry and Ella’s wedding (The wedding was for both the blushing groom and the frightful bride.)
Lulu and Lola’s new set of nose rings (The set was too expensive for either one alone, so Lulu and Lola each paid half and agreed to an every-other-week wearing schedule.)
Roger and the superspy’s secret (Roger told it to the superspy, so now they’re sharing the secret, which concerns doughnuts and explosives.)
If two people own things separately, as individuals, use two apostrophes:
George’s and Martha’s teeth (He has his set of teeth — false, by the way — and she has her own set.)
Lulu’s and Gary’s new shoes. (She wears size 2, and he wears size 12. Hers are lizard skin with four-inch heels. His are plastic with five-inch heels.)
Eggworthy’s and Roy’s attitudes towards dieting. (Eggworthy doesn’t worry about cholesterol. Roy monitors every scrap of food he eats.)
Remember that an apostrophe shows ownership. Don’t use an apostrophe when you have a plural that is not expressing ownership. Here are some examples:
Right: Bagels stick to your teeth.
Wrong: Bagel’s stick to your teeth.
Also wrong: Bagels’ stick to your teeth.
In two special cases, apostrophes do show up in plurals. If you’re writing the plural of a lowercase letter, you add an apostrophe and an s. To help the reader along, you should italicize the letter but not the apostrophe or the s.
If you’re writing the plural of a word used as a word (not for what it means), italicize the word and add a nonitalicized s (with no apostrophe). If you’re writing with a pen, not a computer, italics aren’t possible. Pen-writers should place the plural of the word used as a word or the letter in quotation marks and add an apostrophe and an s. Take a peek at these examples:
You have too many f's in that word, young lady!
The boss throws impossible’s into every discussion of my raise.