I fear this is one of those grammar strictures that’s broken so many times, it’s about to get dumped. Somewhere along the way, editors stopped doing their duty and allowed writers to hang an “s” onto possessives ending in “s,” so they work just like possessives that don’t have an “s” at the end.
The rule, in case anyone still wonders, is this:
Add ’s to possessives that don’t end with the letter “s” (except for “its”). Possessives that end with “s” simply take an apostrophe. For example:
Lt. Valeris’ alacrity enabled Star Fleet to deduce Ambassador Nanclus’ role in the assassination of Klingon Chancellor Gorkon.
In recent decades, the Associated Press Stylebook (a favorite of mine) allowed for the addition of ’s to possessives that end in “s” when the word is only one syllable. Therefore:
Mills’s educational excellence is enhanced by its sylvan campus.
Where do you stand on slipping in a second “s”? (Here’s what Grammar Girl suggests.) In my case, growing up with one rule means that when I encounter examples like the one above, I lose track of the point of the sentence and stop while my mind corrects the grammar. Plus, to my eyes, it looks wrong, that row of “ssssss,” like a cartoon-balloon for a snake. What do you think? Am I being too possessive of the old rules? Is the new usage more helpful? Does it make more sense?