possessive apostrophes and names ending in "s"

The question came up recently over at Jeff Hodges's blog: Hodges' or Hodges's? I gave Jeff the following rule, which I had to look up myself long ago:

Add apostrophe-s to modern names ending in -s, and add just an apostrophe to ancient names ending in -s.

Examples of modern names might be:

Jeff Hodges's blog

the Williams's cat

Gus's problem

For ancient names:

Moses' precepts

Jesus' disciples

Xerxes' armies

Neat, clear, sensible. I told Jeff that I thought CMOS (The Chicago Manual of Style) would back me up on this, but that I'd have to check. It took a while, but I finally did check, and CMOS backs me up only partially. Here's what it says (in part):

7.19: Possessive of names like “Euripides”

Classical proper names of two or more syllables that end in an eez sound form the possessive in the usual way (though when these forms are spoken, the additional s is generally not pronounced).

Euripides’s tragedies
the Ganges’s source
Xerxes’s armies

Note the above is useless for determining what to do with an ancient name like Tacitus. Anyway, for modern names, CMOS agrees with what I said. But regarding ancient names, this is not what I told Jeff. This got me wondering whether I had hallucinated the rule, so I did a wee bit of digging to see whether I had simply pulled the rule out of my ass. Fortunately, I discovered that the rule I'd cited can be found all over the place, so I'm not crazy. Look up "possessives with ancient names" on Google, and you get these results. Note how many agree, just in the top ten search results, that you use an apostrophe only, not apostrophe-s.

So I was wrong to think CMOS would back me up on this, but the rule does exist and is cited on many sites online. So which style guide are you a partisan of? Because that's what this may come down to: the style guide you trust the most.

Over at Jeff's blog, commenter Carter Kaplan weighed in, flippantly noting, "I haven't looked it up, but in English we do what we want to do." Painfully, I have to admit there's some truth to this, and the result is almost inevitably sloppy, stupid-looking English. When a body has no skeleton, it is mush. Structure matters, like it or not, and while I grant the structure of any given language is always changing over time, just as creatures evolve over time, there is nevertheless something that perdures, giving the language shape and coherence. So you can't just do whatever you want to do, linguistically speaking. Kaplan also seems to lean toward Hodges' as the correct possessive, based on his notion of euphony, but CMOS begs to differ here, too (as do I), suggesting that adding apostrophe-s covers most cases: hence Hodges's. Meanwhile, if a name—usually French—ends in a silent "s," then you're still supposed to add apostrophe-s, but you pronounce only one of the two final "s"es. Example: Descartes's philosophy. Pronounce the possessive \deɪˈkarts\.

So: I stand by the rule I cited, but I was wrong to think CMOS would support me.