Jones' is not right, but it is very, very common.
Let me refer you to this very old discussion topic:
Last Names and Plurals
In it I say:
I've also seen the plural and possessive spelled incorrectly nearly as often as I've seen it spelled correctly. When the name in the singular already ends in an S, it is better than even odds that it will be wrong.
Sing. Poss. Jones's
Plur. Poss. Joneses'
And (for the sake of someone who's obviously putting a lot of effort into mastering the language) we should also note that names ending in "Z" follow the same rules. My sister is now a "Schultz" and even her family gets it wrong on occasion.
What I have heard and read (I forget where) is that the pronunciation of the second syllable is "optional," in which case all three sound like the original "Jones." But that's a recent trend, and I doubt it's officially sanctioned. I always pronounce the second syllable.
I also pronounce the second syllable. I don't find it clumsy at all, as some claim. (Jesus's. Moses's. Xerxes's.)
Here is a link to Strunk's Elements of Style where Strunk discusses the 's. Just so you know it isn't just my opinion.
Strunk, Rules of Usage, ELEMENTARY RULE 1 Since it is Strunk's Elementary Rule 1, reinforced by his citation of the United States Government Printing Office and of the Oxford University Press, it is a wonder why so many pundits are confident it isn't so.
Maybe I need an update.
Glenn said: Maybe I need an update.
I think not. I trust what I read here more than I do most forums. That second syllable does get "clumsy" at times, but I would no more omit it when speaking than I would when writing.
Found an interesting thread on the topic here: http://forum.wordreference.com/showthread.php?t=1438256
Also learned in that thread that I often tend to do what they called elision (the intentional slurring of that second syllable … had to look that up). The consensus seems to be that some English dialects, and many English-speaking pundits, do not pronounce the second syllable. So it seems to be a matter of style. Nonetheless, I'm quite confident we are pronouncing the words as they should be.
It may be style. But if somebody sasses or obsesses, declaring there are too many sibilant sounds too close together, don't fret. If he reassesses, he will see the light. Every speaker of English possesses the ability to string lots of sibilant sounds together.
So, if this kind of objection prepossesses you, free your mind. Several common words have three sibilant sounds in close proximity, and are quite similar to
Jesus's (Jesus's recesses were short and few.)
Moses's (Moses's processes were veiled.)
Xerxes's (Xerxes's lieutenant reseizes control of his region.)
Maybe you can think of a few. Let me get you started:
Many many more … but why waste bandwidth? :)
I was trying to show that different sibilants, as well as acronyms, can force elision. I liked your "Xerxes's" example, and tried to expand on it. Have we missed any sibilants? I suspect S, X, and Z pretty much exhaust the possibilities. The challenge is now yours to find any (if any) we've missed. And a "soft C" doesn't really count, because it's still the S sound, but here's one anyway: services' (that which belongs to the armed forces).
Since there are three species, one could grow multiple sassafrasses. One could feed the leaves to their asses.
I own controlling interest in three businesses.
Many schools have several buses to transport their students. (BTW, my faded memory of Deutsche translates students into studentenen.) A computer can have multiple busses.