Let’s review possessives. Keep in mind I’m a Chicago gal (as in Chicago Manual of Style) so I use their conventions. If you use a different style guide, you can find those guidelines in your manual.
Michael Jones owns a car. It’s Michael Jones’s car. (Add the ‘s. You say it when you speak, so type or write it, too.)
Michael and Sarah Jones own a house together. It’s the Joneses’ house. (Joneses is the plural of Jones. Add just an apostrophe, because plural possessives don’t take the additional S.
Michael’s work is Mr. Jones’s job. Sarah’s is Mrs. Jones’s job.
And I’ll bet they have separate toothbrushes, so there’s Michael’s and Sarah’s toothbrushes. BUT, they probably own the TV in the parlor jointly, so that’s Michael and Sarah’s TV. (Or Sarah and Michael’s TV. Let them sort that out.)
If they have a friend named Jesus Garcia, and he’s got a car too, that’s Jesus’s car. If you’re talking about the Biblical figure Jesus, you don’t add the S; that’s considered a “classical or historical name,” and those take just the apostrophe. Moses’ tent. Xerxes’ troops. Jesus’ birth.
And I’ll leave it at that. If you have questions, comment and I’ll respond as I have time. It’s copy-editing day here.
Grammargeddon! 
Hello Karen!
I have a question about possessives.
Would you say:
They lost their brain or they lost their brains.
They have one brain each, no? But the common usage seems to favour plural. What’s the rule?
Thank you!
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Hello, Lucie!
There are multiple individuals, and each one presumably has a brain. That means multiple brains, right? “They lost their brains.” Rather like “they lost their minds.” If this is SF and there’s a collective situation, like the Borg or similar aliens, perhaps you could make a case for them sharing a brain. Rather like conjoined twins who share a brain, if you see what I’m driving at. (This isn’t possessives per se, but it’s agreement regarding pluralism. The possessive in either case is “their.” See what I’m saying?)
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Hey Karen!
I see what you’re saying. It’s just that, in French, we would say “their brain” or “their mind” as a rule, because people usually only have one each. I was wondering if the rule applied in English, too.
People often make the mistake of using plural in French, so I was not sure if if it was a mistake in English, too.
Thanks for clarifying!
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IF (and that’s a big IF) you’re using the singular they (also called “epicene they,” about which I’ve also written here on the blog), then of course there would be only one brain under consideration.
Some people prefer the nongendered “they” as their personal pronoun of reference. I’m thinking of folks in the LGBTQ communities, here, and some feminists as well. Rather than “his” or “her” these individuals prefer “their” be used when speaking to/of them.
Something else to keep in mind, to muddy the waters rather than clarify, I’m sure.
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Easy problem for a fiction writer to solve. Never choose last names that end with s. That’s my solution, anyway. 😀
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Cheater. 😉
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