I quote Martin Hewings Advanced Grammar in Use:
p.106 unit 53:
<...>
To make the possessive form of names ending in -s (pronounced /z/)we can add either ' or 's:
Ex.:It's Derek Jones' new sports car or It's Derek Jones's new sports car.
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I know that 's could be used but I've never come across it before. I believe it's custom to use just ', but really, how usual is this other unusual way to write it?
Pontus wrote:but really, how usual is this other unusual way to write it?
Apparently J.K Rowling uses it systematically in Harry Potter 5. I can't remember how it is in the first 4 ones.
Captncavern says it's disappearing. Then it makes sense that she uses it in Harry Potter. Old fashion grammatical forms fit well with the magical atmosphere, I think.
Penn ar Bed
The end of the land
Le commencement d'un monde