Possessives and Attributives

Q. It just occurred to me that “Achilles’ heel” is wrong, according to CMOS 7.17. It should be “Achilles’s heel,” right?

A. Technically, yes: “Achilles’ heel” is contrary to Chicago style, which would call for “Achilles’s heel.” CMOS 7.19 addresses the issue directly: “Classical proper names of two or more syllables that end in an eez sound form the possessive in the usual way (though when these forms are spoken, the additional s is generally not pronounced).” For example, “Euripides’s tragedies.” But like Achilles’s mother, we failed to cover “Achilles’ heel,” a term that therefore remains vulnerable to stylistic ambiguity. Thankfully, Merriam-Webster is there to shield us from the arrows of editorial uncertainty. We defer to that resource and consider “Achilles’ heel” as an established exception to Chicago style.