Until a while ago, I would think that both forms were OK, and meant the same thing. But someone said that just one was right, think they meant Jones's.
Now, which is really true?
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.
Jones
Pl. Joneses
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.