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