But times have changed. Today many people and organizations don’t observe the apostrophe-only rule. Not even The New York Times, where I spotted “Degas’s.”
Leave off the s for the possessive
Grammar Girl says opinions are divided, but she prefers to leave off the s.
Here’s what my old AP style guide says about the possessive and singular common nouns ending in s:
Add ‘s unless the next work begins with s: the hostess’s invitation,the hostess’ seat; the witness’s answer, the witness’ story.
In another complication, the Purdue Online Writing Lab (OWL) says:
add ‘s to the singular form of the word (even if it ends in -s)
OWL’s rule means that you’d write about “General Mills’s divisions.”
I think the AP and OWL recommendations are too complicated. Let’s keep things simple! However, if you prefer different rules, it’s okay as long as you apply them consistently. Consistency will make it easier for your readers to process what you write.
Note: I updated this post on Dec. 27, 2015 by adding links.