this is often problematic i've read these guidelines carefully several times and there is a specific reference to the name williams in the second one but it's not a hard and fast rule just a description of customary usage it seems that in both guidelines the way the possessive form sounds best when pronounced is a good guideline see mos poss and apostrophe possessive apostrophe particularly these sections basic rule singular nouns singular nouns ending with an s or z sound see the second paragraph after the two bulleted items for specific discussion of the name williams would you say williams' wife or williams's wife with the second one pronounced williamses wife i don't know which one i would say i've never known anyone with that last name there is a small consideration that williams' wife sounds the same as william's wife with the second example being a man whose first name is william but i don't know if that's a problem especially in an article where it is clear that it is the man's last name with williams's wife it is clear beyond a doubt that his last name is williams what do you think