How do I use a possessive apostrophe when the word ends in 's'?

Apostrophes can be trying at the best of times, but when the word ends in 's' it gets particularly confusing. How do you know where to put the apostrophe? And what about an extra 's'. Do you need it? The simple fact is that tastes in punctuation have changed over time and now we are left with a headache as we try to follow the rules we were taught all those years ago (well, it was a wee while ago for me, but perhaps not for you).

When the possessing noun is plural and ends in 's' just put the apostrophe after the 's' and then move on. (Some plural noun examples, teachers, shopkeepers, sisters, bosses, but not firemen as although it is plural it doesn't end in 's'.)

All the teachers' pupils ran the cross country run today. Not one of those teachers accepted an excuse. Every child had to run.

The shopkeepers' concerns were unfounded. None of the shopkeepers needed to worry about their shared concerns, after all.

When the sisters' chores were done they were free to annoy their little brother. Those sisters had too much time on their hands. More chores, I say.

The bosses' combined pay would have kept me in champagne for the rest of my life. Are those bosses really worth all that money?

So far, so good. But when you are writing a person's forename or surname things can vary a little and get vague.

The name Jesus and names from the ancient world that end in 's' just take an apostrophe and no extra 's'. I should mention that some people (and countries) include other biblical names in this rule too. Some don't. Examples of names that fall under this rule are Archimedes, Achilles and, well, Jesus.

Hercules' labours were laborious. Well, they must have been or they wouldn't have been called labours, would they?

Names that end in an 'iz' sound (that's called a voiced 's') need an apostrophe and then an 's' added.  

Les's friends never know where to put the apostrophe in his name. They were pretty certain it wasn't Le's.

Names that end in 'ss' need an apostrophe and an 's'.

Mr Moss's house was painted moss green.

But, if you are writing about the Moss family's house you would drop the extra 's' because (...see the first rule at the top) it would be a plural. Like this:

The Mosses' house was painted moss green. Some people like moss green.

So what about all the other names that end in 's'? Lynne Truss, in Eats, Shoots & Leaves refers to Fowler's Modern Usage and offers the following:

...modern names ending in 's' (including biblical names, and any foreign name with an unpronounced final 's'), the 's' is required after the apostrophe.

Lynne Truss's book is an invaluable resource on apostrophes. Yes, indeed!

She used Alexander Dumas's name as an example in her book. I stole that one because I couldn't think of another foreign name with a silent final 's' off the top of my head.

This whole business is messy. It is complicated. Don't worry if your head hurts a little just thinking about it. But next time you need to place an apostrophe just re-read those rules and place that apostrophe with confidence.