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 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?   "