Actually, there's some dissention over the rules governing punctuation with possessive nouns in recent years. For the most part, there's been a movement to simplify things from the traditional approach.
My understanding is that, traditionally, the rule has been that all singular nouns are made possessive by the addition of
's to the end. Even singluar nouns like "Chris" that already have an
s (Chris's).
In recent years, the move has been more toward simplifying the rules to make singular nouns ending in
s fall in with plural possessive nouns and just add an
' after the already-present
s (Chris'

. As you might imagine, it's not the grammarians of the world who are championing the simplification route!
Of course, there's also the more complex and hard-to-remember version that relies on the sound and syllable situation with the word! I just clipped this explanation from
www.grammarbook.com which explains it pretty well:
To show singular possession with proper nouns ending in s or an s sound, let the pronunciation guide you. If a new syllable is required to say the name in possessive form, then add an apostrophe plus s.
Examples Ms. Jones's office is next to Ms. Straus's.
Texas's weather in summer can vary greatly.
Mr. Hastings' hat blew off in the storm.
The possessive form of Hastings does not require an extra syllable.
Example Mrs. Lees' hair showed dark roots; it was time for an appointment with her hairdresser.
The name of this person is Mrs. Lees.
Fun stuff, huh?
Anyway, the rules are kind of in flux depending on who you ask. This little piece of punctuation rule is sort of fluid right now. My advice would be not to worry too much about it unless you're in a "high stakes" situation. If you're taking a class or aiming for publication, find out which set of guidelines the instructor/institution/publication prefers.
If anyone has a better understanding of the groups fighting this fight (funny image, there--guidebook-wielding grammar czars fighting off the lazy populists and their erasable pens), I'd love to hear about it.
Am I the only one who gets kind of excited thinking about how we're having an honest-to-God metamorphosis in not just our usage but our punctuation rules like this? It's pretty cool, I think.
Or maybe I just need to get out more... :unsure: