When talking about grammar, the addition of an s to a singular proper name would be awkward to pronounce, so the additional s does not have to be added (it is optional) *after* the apostrophe - for example:
If Mrs Jones owns a handbag, you can say:
"This is Mrs Jones' handbag."
or
Mrs Jones's handbag
but never Mrs Jone's handbag (unless the surname is Jone)
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Proper Noun - plural
Mr & Mrs Hess bought the house next door.
The Hess' [more than one Hess, plural] have invited us to their house-warming party!
The three Ross brothers all play football.
The Ross' [more than one Ross, plural] are well-known in our community.
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