The very first rule of the very first page of the very first section of The Elements of Style, William Strunk Jr. and E. B. White’s classic book on writing, concerns apostrophes. This is a punctuation mark that puzzles many, but from this fog of confusion, Strunk and White distill a single, vital droplet of information: “Form the possessive singular of nouns by adding ‘s.” The first example they cite is “Charles’s friend.” That’s right. Do you hear me, world? If it ends in an s, it still requires an apostrophe, then another s.
This works in 99.9% of cases (this number has been scientifically discovered . . . in my head). There is, however, one weird exception: “for goodness’ sake,” and other “sake” expressions in which the first noun ends in s. The Chicago Manual of Style (sixteenth edition) justifies this as creating “euphony.” Which is to say, it sounds better.
Interestingly, in CMoS-land, there used to be other exceptions—special rules about names ending in a silent s or names ending in an “eez” sound (CMoS cites “Xerxes” or “Euripides” as examples, though I prefer to think of this as the Jeffrey Eugenides rule). In the sixteenth edition, however, the writers changed their minds. Now Jeffrey Eugenides gets to own things just like everyone else: “Jeffrey Eugenides’s beer cozy.”
Even more interestingly,* Strunk and White and CMoS do differ on the issue of Jesus. Not in a religious way, mind you, but in a grammatical way. Which I think we all know is what really matters. Strunk and White advocate for writing, say, “Jesus’ clown makeup.” CMoS says “Jesus’s clown makeup.” But then, something tells me that the Insane Clown Posse does not understand the mystery of apostrophes…or magnets.
*Provided you are boring.