What is plural possessive?
Is formed by adding an apostrophe and an s or just an apostrophe after an s to a noun in a plural form.
First Rule: First, we have to make the noun plural before we could show the plural possession in a noun. Then use the apostrophe after the s if the plural form calls for an s .
one baby, two babies, two babies' diapers
one dog, two dogs, two dogs' shampoo
one fox, two foxes, two foxes' lairs
Second Rule: If the plural form changes the noun that doesn't end with an s , then add an apostrophe and an s .
one man, five men, five men's watches
one sister-in-law, three sisters-in-law, three sisters-in-law's gifts
one child, two children, two children's bicycles
one cactus, two cacti, two cacti's leaves
Third Rule: If the plural form of the noun is the same as the singular form, add an apostrophe and an s just like with the singular form.
one fish, two fish, two fish's adventure
one sheep, two sheep, three sheep's wool
Fourth Rule: For the proper noun, follow the same rule as to the common noun.
Singular : Mr. Williams bought a puppy.
Singular Possessive : Mr. Williams's puppy barked at the kids.
Plural : The Williamses traveled to Europe for a week.
Plural Possessive : Their neighbor, Mrs. Adam, took care of the Williamses' puppy.
Singular: Mrs. Adam cooked dinner for her son.
Singular Possessive : Mrs. Adam's stew is a winner.
Plural: The Adams bought a new house.
Plural Possessive : I attended on the Adams' house blessing.
Source: Modules from my online grammar class. :)