English Language & Usage 
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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.
The basics.
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.
The Advanced.
What if the possessing noun is a conjoined phrase like “my wife and I”?
Kosmonaut gives an excellent answer to this question.
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:
Happy Mothers’ Day or Happy Mother’s Day
Beginner’s or Beginners’ Guide
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.
Filed under Orthography
Tagged: possessive-apostrophe
Admirable. Everything needed in one place. Just one niggle: all your apostrophes before the final section (The Advanced) are turned — left single quote /‘/ instead of right single quote /’/.
Yeah, some weird WordPress thing. I think I fixed them with straight quotes. Thanks.
That’s probably a quirk of WordPress. The apostrophes come after some HTML (because the words are underlined) so WP interprets them as opening quotation marks. There’s probably some way around it.
TRiG.
The apostrophe came into English in the mid-sixteenth century, and has caused trouble ever since. Even now, anomalies abound. There is one underground station in London called Earl’s Court and another, two stops away, called Barons Court. My son went to St Edmund’s School, but my Oxford college is St Edmund Hall.
The post sets out to cover only the possessive apostrophe, but it’s perhaps worth saying there are two further uses. One is to indicate missing letters in contractions such as ‘don’t’. The other is to make plurals where its absence might be confusing, as in ‘if’s and but’s’ and, more controversially, in ‘1970’s’ and ‘CD’s’.
There is a case for abolishing the apostrophe altogether, one which Peter Harvey makes here: http://lavengro.typepad.com/peter_harvey_linguist/2008/11/apostrophes.html
I wish you would not try to tell people that the apostrophe makes an “iz” sound. It really does not.
I didn’t tell people that the apostrophe makes an “iz” sound, or any other sound for that matter. Apostrophes don’t make any sound as far as I’ve ever understood it.
I think Geoffrey Pullum said somewhere that the apostrophe was the 27th letter of the alphabet.
Shaw eliminated apostrophes (which he called “uncouth bacilli”) from his texts, and I’ve never missed them. He also used l e t t e r s p a c i n g instead of italics for emphasis, which takes a little more getting used to.
I’m surprised you didn’t include the possessive “its”. It doesn’t have an apostrophe at all! It took me years to differentiate it from “it’s”, when I realized it followed the same form as “his” and “hers”.