Apostrophes are lovely little critters, but they tend to boggle the mind if you think about them too much. One of the most common questions on EL&U regards proper usage of an apostrophe to indicate possession.
How do we use an apostrophe to indicate possession?
If the possessing noun is singular, add an 's (apostrophe-s).
Sara's beast friends were all balrogs.
If the possessing noun is plural and ends in s, add an ' (apostrophe).
The beasty balrogs' game was very fun.
Well, now, that’s pretty straightforward, right? Except that apostrophes have this annoying habit of jumping into your brain and scrambling your thoughts. There are lots of ways to get confused.
What if the possessing noun is plural and does not end in s?
Then treat it like the singular case, and add an 's (apostrophe-s).
The children's books were tucked away in their cubbies. The geese's honking alerted the dog to the fox’s presence.
What if the possessing noun is not plural, but ends in s?
Well, golly, it turns out this one is complicated. Generally speaking, these are treated just the same as other singular nouns:
The glass's rim was cracked.
But this has not always been the case. Historically, names ending in s followed the plural rule:
*Seamus' writings were well-known throughout Galway.
For proper nouns, this is considered a stylistic choice, but following the singular form is more common these days:
Seamus's writings were well-known throughout Galway.
You’d think with just four rules (which are really just two if you think about it) that noöne would have much trouble with possessive apostrophes. But those apostrophes sure are pernicious.
What if the possessing noun is a conjoined phrase like “my wife and I”?
Those rules are all well and good, but how do I decide whether the possessing noun should be plural or not in the first place?
There are a lot of questions about this very sticky wicket on EL&U. Some examples are:
Does the guide belong to one user or many users? Is the day for one mother or all mothers? Either way is technically acceptable, but generally speaking, we consider a single instance and an abstract entity. So one copy of the guide for one abstract generalization of user means we usually say “User’s Guide.” Mother’s Day is trickier because we could celebrate all mothers on that day, but it is supposed to be a day on which we honor our own mother, so “Happy Mother’s Day” unless you have two mommies.
Finally, we see that possessive apostrophes are disappearing for plural nouns that demonstrate affiliation, so it is acceptable practice to use phrases like “User Group” instead of “Users’ Group.”
That is a little summary of possessive apostrophes, along with some fun links for further reading.