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Singular they rule
(pawb.social)
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Does it? If multiple people left multiple wallets on the table, it would be, "They left their wallets on the table." Multiple people can't really leave a single wallet behind. Or at least that would be very unusual and unintuitive.
True. I'll change it to a (hopefully) better example.
“A customer forgot to pay their bill” sounds totally normal to me though, you just need a reference before throwing a pronoun out there, if the context doesn’t clear up the number of people referred to.
I agree.
This sounds normal to me, how else would you word it?
If it was a singular customer, I would say "He forgot to pay his bill" (or she/her depending on the gender).