I just received an email from Heather, who is desperate to understand correct apostrophe use for possessives. Relax, Heather. It isn't as tricky as it seems. We can cover the basics in just four simple rules.
1. Generally, we do not use apostrophes to make a plural (that is, to indicate more than one). That is why these examples are all correct without an apostrophe:
The defendants agree to the settlement.
The citizens have not forgotten what happened.
The boys need a ride home from the game.
2. To make a singular word possessive, add an apostrophe and an s:
This defendant's attorney has left a message. (one defendant)
The citizen's request was quite reasonable. (one citizen)
The boy's father picked him up already. (one boy)
The boss's flight will arrive at 4 p.m. (one boss)
3. To make a plural word possessive when the plural ends in s, add just an apostrophe:
Both defendants' attorneys edited these notes. (two defendants)
The citizens' lobby has become huge. (many citizens)
The boys' fathers picked them up already. (two or more boys)
4. To make a plural word possessive when the plural does not end in s, add an apostrophe and an s.
The children's reading room is colorful. (children = plural of child)
The alumni's contributions kept the college afloat. (alumni = plural of alumnus)
The men's club has disbanded. (men = plural of man)
Those are the basic rules. But Heather also wanted to know what to do when two entities possess something. Should both names be made possessive? Do both names need an apostrophe?
That depends. If they share the thing they possess, they share the apostrophe. If each person has his or her own, each also has his or her own apostrophe.
My mother and father's home is in Florida. (They share the home–they share the apostrophe.)
Davey's and Ella's toothbrushes are already packed. (Each person has a toothbrush–each has an apostrophe.)
The girls' and boys' teams are both in the playoffs. (Both girls and boys have their own team–both have their own apostrophe.)
The girls and boys' team is excited about being in the playoffs. (One team of girls and boys–one apostrophe.)
We could make the rules slightly more complicated if we wanted to. For example, we could think about the very few instances in which plurals are formed with apostrophes. We could talk about the Associated Press style for forming the possessive when a singular proper noun ends in s (Chris'). But let's not do that today. Instead, let's keep it simple and begin to enjoy our weekend.
If you want to master punctuation, take my online self-study course Punctuation for Professionals.