This is MY BIGGEST PET GRAMMAR PEEVE. I do this for a living, so everyone pay attention and do it right:
Plural: Dogs, cats, cars, etc.
Single possessive: Dog’s, cat’s, car’s, etc.
Plural possessive: Dogs’, cats’, cars’, etc.
BUT: Person’s and people’s, woman’s and women’s, man and men’s, child and children’s
HERE’S WHERE IT GETS TOUGH –
Single possessive of a noun or proper name ending in “s”: bass’s (NOT bass’), Jones’s (NOT Jones’), Ramirez’s (NOT Ramirez’). Example: “Mr. Jones’s attorney has filed suit against Mr. Ramirez’s company.”
Plural of a noun or proper name ending in “s”: Joneses, Ramirezes, losses, masses. Example: “Masses of people are counting their losses after trying to keep up with the Joneses.”
PLURAL POSSESSIVE of a proper name ending in “s”: Joneses’, Ramirezes’. Example: “The Ramirezes’ dog just peed on the Joneses’ new Lincoln Navigator.”
Sources: “The Elements of Style,” by William Strunk, Jr.; and “The Grammar Bible,” by Professor Michael Strumpf