I was (un)fortunate enough to have had an English teacher who was positively anal about apostrophes, so I think I'm generally fairly good about using them correctly - except when I'm being lazy about reading something before I post it.

The one thing we were taught which differs from what I've read other people state is correct, is the use of apostrophes in the possessives of personal names that end in "s". We were taught that for biblical and classical names the final "s" should be omitted, but for all others it should stand. Hence, I was told, one should say Hercules' big head and Jesus' dog, but Charles's big head and Francis's dog (unless you're talking about St Francis maybe...??).

I've always tended to accept (if not always follow) this rule as it was drilled into me, but now I don't know if it's just a widely accepted quirk, one of those pedantic little quibbles that's become outdated, or simply a personal either/or decision!

A random flick through some mythology tomes on my bookshelf doesn't help as I've found examples both using and omitting the final "s", and my Word spell/grammar check cheerfully allows a word either with or without it too. This is probably a very trivial and nit-picky question but it's been bugging me since I thought about it!