Two weeks ago the question was posed: What to do with proper nouns that end in 's' when making them possessive? This is a conundrum. It is quite pertinent to me, my last name ending in 's'.
Well, it turns out that there is a raging debate over this very question in the punctuation world. You may be surprised (or completely unsurprised) to learn that there is always a fierce debate of some kind among grammar/punctuationphiles. Probably because many grammar and punctuationphiles are impossible-to-please sticklers.
Perhaps because this debate has little hope of ever being solved, you have options! Meaning that no matter what you do, some people will think you are ignorant, others will laud you for using correct punctuation in a punctuation-ignorant world. And, I suppose we should admit that most will not care one way or the other and some may not even know what an apostrophe is.
Laura, a former proofreader (I'm so jealous that you had that job!) and commenter said that, according to AP format, a singular possessive proper noun that ends in 's' should have an apostrophe after the 's':
James' car. (Please correct me if that's not what you meant, Laura!)
However, according to the Purdue Online Writing Lab, either "James's car" or "James' car" is acceptable. Choices! Just go with what strikes your fancy! Isn't punctuation fun?!
For plural possessive proper nouns (say that three times fast), put the apostrophe after the 's':
The Joneses' car
I'll admit, that looks clunky but what should the Joneses do? Change their names? No. I do not think that they want to stand in that social security office line for the sake of avoiding an awkward plural possessive!
According to Eats, Shoots and Leaves, there are various disagreements over the above rules throughout the punctuation world and it's best not to get too worked up. The author states that the only consistency she found in the dozen or so punctuation guides she consulted was that they used the poet John Keats' name (or John Keats's name!) to illustrate their various and sometimes opposing points.
So, go crazy and say Keats' or Keats's but whichever you choose, be consistent. (And when I say crazy, I mean crazy within the bounds of punctuation decorum, of course. Don't go writing "Ja'mes car" or anything like that. Even people who don't know what an apostrophe is will recognize that you're not the brightest crayon in the box.)