Possessives: nine-tenths of the law?

The Supreme Court's 5-4 decision in Kansas v. Marsh revealed a deep divide over ... when to use the apostrophe-S in possessive nouns that end in S, and when to just use the apostrophe.

The majority opinion — written by Clarence Thomas and joined by John Roberts, Samuel Alito, Anthony Kennedy and Antonin Scalia — referred to Kansas' statute. In dissent, David Souter, joined by Stephen Breyer, Ruth Bader Ginsburg and John Paul Stevens, referred to Kansas's statute. Scalia wrote a separate opinion concurring with Thomas on Marsh but found a middle ground on the additional S. He wrote Kansas's, Ramos's and witness's, but Stevens', Adams' and Tibbs'.

An article at Legal Times discusses the ramifications, along with this note on usage:

By a margin of 7-2, the strict anti-s view appears to be the clear preference of the land's highest court. Yet experts on American usage overwhelmingly agree that Souter's approach is the only one that is proper. As explained by Bryan Garner, author of A Dictionary of Modern American Usage, most authorities on the subject recognize only two types of singular nouns for which it is acceptable to omit the additional s: biblical or classical names, such as Jesus, Moses, or Aristophanes, and nouns formed from plurals, such as General Motors or Legal Times. (Journalists are often more liberal in excluding the additional s, but that is typically based on the pragmatic goal of conserving print space rather than on any ideological grounds.)

and this editor's note:

Legal Times admits to following Associated Press style, which omits the s after the apostrophe in creating possessives of all singular proper names ending in s, not just biblical and classical names.

Omitting the S may be an Associated Press Stylebook thing, but it has certainly caught on, as Bill Walsh pointed out in May: When a Washington Post headline included the word Roberts's, people complained. But that's Post style; the paper bucks the AP tradition (as do the New York Times and the Wall Street Journal).

Here's the relevant rule from Garner's:

To form a singular possessive, add -'s to most singular nouns — even those ending in -s, -ss, and -x (hence Jones's, Nichols's, witness's, Vitex's).

Here's the relevant AP rule:

SINGULAR PROPER NAMES ENDING IN S: Use only an apostrophe: Achilles’ heel, Agnes’ book, Ceres’ rites, Descartes’ theories, Dickens’ novels, Euripides’ dramas, Hercules’ labors, Jesus’ life, Jules’ seat, Kansas’ schools, Moses’ law, Socrates’ life, Tennessee Williams’ plays, Xerxes’ armies.

But it looks as if the Supreme Court doesn't have an in-house stylebook to help settle these disputes. For a small fee, I'd be willing to offer my services.

Related:
An oft-forgotten style rule [A Capital Idea]
Obsessive over possessives [A Capital Idea]