Apostrophes, plurals, and the pesky letter S

I can’t focus on my manuscript, which is, among other things, about ADHD. Stop laughing.

But I hadn’t forgotten that some of you asked about whether you add an apostrophe s in a word that ends with an s or a z. Chicago has gone back and forth about these issues over the years and editions. I started to write out the quote, but then I remembered, you don’t care. You are staring at your computer yelling “Indy, if we wanted to read Chicago’s version, we would have bought the damn book.”

Right, sorry. Here it is:

Chicago’s definitive rules on how to add apostrophes to words with “s” sounds at the end

1. It is now acceptable, nay, required, to add an s at the end of words that end with an s or z. Sorry folks.

2. Add an apostrophe s to words where the s is silent.

3. Add an apostrophe s to proper names that end  in an s

Kansas’s legislature

but Lincolns’ marriage [this is different because “Lincolns” is plural]

Decartes’s three dreams

Euripides’s tragedies

(Source: Chicago Manual of Style, 16th ed., 7.15–18)

Exception 1. If the plural and the singular is the same, do not add an s after the apostrophe (e.g., politics’ true meaning or United States’ role; CMOS 7.19).

Exception 2. This one I have to quote because it is really funny “For the sake of euphony a few for . . . sake expressions used with a singular noun that ends in an s end in an apostrophe alone…for goodness’ sake” (7.20)

This is a change from Chicago 15. But it’s the rules for standard American publishing. Now you know.