Is it "Edwards' advice" (one S) or "Edwards's advice" (...S'S)? The first one is correct.
OK, then, is it the hostess' advice or the hostess's advice? This time, it's the second one.
No wonder everyone's confused, right? I gave you two opposing answers, but both are correct.
The key point is this: "proper nouns" (names) versus "common nouns" (not names). It's "no" on the extra S for names but "yes" on the extra S for ordinary nouns.
My way of remembering this is to remind myself that people get to spell their names any way they wish. So if Marie wants to spell her name Maree, that's OK. It's her prerogative.
In my experience, companies are very touchy about their names -- understandably so. It seems to me that most tend to leave off the extra S, which happens to be in keeping with AP style. I feel the same way. I myself don't like the look of "Edwards's advice." I don't know why -- I just don't like it. I prefer "Edwards' advice."
So ... names don't have to follow the standard rules -- that's how I remember it. My personal suggestion would be to ask the person whose name you're making possessive, if they are available to you. If not, go with the AP rule. If it's a company's name, I'd look on the Web site for press releases and see how the company chooses to handle it. It really is their choice.
Now for the exception: Even with a common noun ending in S, *don't* add the extra S after all if the next word begins with S. Under that rule, these are all correct:
- the witness's answer
- the witness' story
- the hostess's advice
- the hostess' seat
Don't ask me why it's like this. That's a question for Grammar Girl.
Does everyone agree on this rule? No.
Strunk & White is at odds with AP style on a few points, including this one. So are a few other style guides.
Grammar Girl is with The AP on this one, I noticed. But she, too, acknowledges there's no agreement among the experts.
Some guides extend the exception to words that end in ce, x and z. The AP doesn't.