- Add 's to singular nouns not ending in s: the church's members, the girl's parents, Xerox's profits.
- Add 's to singular common nouns ending in s unless the next word begins with s: the bus's engine, the bus' seats, witness's answer, the witness' story.
- Use only an apostrophe for singular proper names ending in s: Drakes' decision. And add only an apostrophe to plural proper names ending in s: the Parkses' home.
- Add 's to plural nouns not ending is s: children's passes, men's bike, women's rights, women's room.
- Add only an apostrophe to plural nouns ending in s: the girls' books, boys' bike, plants' supervisors, families' cars.
- When a plural noun is possessive but each person "owns" only one item, the item should also be listed in plural form. To confirm correctness, rephrase the possessive relationship as an of phrase:the children's brains or the brains of the children; the teachers' hands or the hands of the teachers.
- Follow the rule above (and its test for correctness) when using plural nouns and possessive pronouns: The children became upset when their mothers left the room or the mothers of the children. Gerry and Lena took their dogs for a walk or the dogs of Gerry and Lena.
- When two or more people jointly own an item, put the apostrophe after the noun closest to the item: Gary and Gina's car (they jointly own car), Gary and Gina's cars (they jointly own more than one car). But when two or more people separately own items, put an apostrophe or an 's after each noun: Gary's and Gina's cars.
- When writing about a family in the plural, add s and then an apostrophe: the Abernathys' Christmas greeting (but Bob Abernathy's Christmas greeting).
- Add only an apostrophe to nouns plural in form, singular in meaning: mathematics' rules, United States' wealth.
Many pronouns have separate forms for the possessive that don't use an apostrophe: yours, ours, his, hers, its, theirs, whose. Use an apostrophe with a pronoun only when the meaning calls for a contraction: you're (you are), it's (it is). Follow the rules listed above in forming the possessives of other pronouns: another's plan, others' plans, one's rights, someone else's umbrella.
- Treat nouns that are the same in singular and plural as plurals, even if the meaning is singular: the two deer's tracks.
Do not add an apostrophe to a word ending in s when using the word as an adjective--describing the following noun. If the prepositions for or by would be more appropriate than the possessive of, do not use an apostrophe: a radio band for citizens, citizens band radio; a guide for writers, a writers guide; a day for veterans, Veterans Day; a union for carpenters, a carpenters union. Add 's, however, when a term involves a plural word that does not end in s: a children's hospital. If you're giving the proper name of an organization or other item, try to respect the style it uses--even if that style differs from these guidelines: the Metropolitan Teacher's Association, The World-Class Speller's Guide.
Follow the rules above for possessive words that occur in such phrases as a day's pay, two weeks' vacation, four years' experience, your money's worth.
Avoid excessive personalization of inanimate objects. Use an of construction instead when appropriate:the rules of mathematics instead of mathematics' rules.
Also, avoid using a possessive name as a plural: The free passes are available at four McDonald's restaurants. Not: The free passes are available at four McDonald's.