It’s Achilles’ heel but James’s foot. Only ancient names that end with an s do not employ a possessive form of ‘s. All modern names do. One way to avoid the confusion is to rearrange the wording: the heel of Achilles but the foot of James. Yes, it adds words, and sounds a little stuffy, but the confusion of the possessive form is gone. (Note: While not all style guides agree, most do.)
View other Tips and Tricks from the Editor.