Q. I thought Chicago style used to say to use only an apostrophe for the possessive of a name like “Harris” that ends in “s.” Am I imagining things?
A. You’re not imagining things, but Harris’ hasn’t been Chicago style since the late 1960s. The first edition of CMOS (1906) advised using an apostrophe alone to form the possessive of “proper names of more than one syllable ending in s or another sibilant”; for one-syllable names, the rule was the same as for names that didn’t end in a sibilant—that is, add an apostrophe plus an s. In other words, you would write “King James’s Version,” “Burns’s poems,” and “Marx’s theories” (one-syllable names), but “Moses’ law,” “Demosthenes’ orations,” and “Berlioz’ compositions” (names with two or more syllables, a category that Harris belongs to). See paragraph 103 in the first edition.
Except for a couple of clarifications for names like Charlevoix and Horace (both of which would get an ’s), the original rule remained in place until 1969, when the twelfth edition was published. That edition eliminated all but a few exceptions to the ’s convention: “Exceptions are the names Jesus and Moses and Greek (or hellenized) names of more than one syllable ending in es”—as in “Jesus’ nativity,” “Moses’ leadership,” and “Xerxes’ army” (12th ed., ¶ 6.8).
The sixteenth edition (2010) then eliminated all remaining exceptions (including one for names ending in a silent s that had been added to the fifteenth edition [2003]), and that’s where CMOS stands today—that is, add an apostrophe plus an s to form the possessive of any person’s name, regardless of number of syllables or ancient pedigree.
So whether you’re referring to “Moses’s leadership” or (to bring things up to date) “Harris’s speech”—or, yes, “Walz’s speech,” though single-syllable names ending in z were never in question—Chicago’s rule for forming the possessive of a person’s name is now the same for all.
For more details, start with CMOS 7.17.