Possessives and Attributives
Q. Which is the correct singular possessive form? “Professor Davis’ class” or “Professor Davis’s class”? My history professor specifically requests our guide be the CMOS. Am I wrong that CMOS promotes both usages in this case?
A. In its 15th edition, CMOS allowed the style shown in your first example, but as of the 16th edition it is no longer recommended, although it is not incorrect and other style guides might allow it. You’ll be safe if you add the s, even if your professor’s CMOS isn’t up to date.