Sioned yw priodferch Alys.
In the newly released iOS app, the only answer marked as correct is "Sioned is Alys's bride". However, "Sioned is Alys' bride" should be correct, as should 'Sioned is the bride of Alys'.
Within the app there is no means of choosing 'My answer is correct' for this question.
The website asks different questions so it wasn't possible to discuss this directly from the question.
You can flag it up, but the only option is "My answer is correct". I would personally only put an apostrophe and not double up on the s.
I did, or tried, but the app didn't provide an option for 'My answer is correct' for reporting - as I mentioned above. Plus, there is never any feedback to acknowledge the report from the (iOS) app, unlike the website, so it is never clear that anything actually happens.
This issue is almost as fraught as the Oxford Comma! In any case, "Alys'" should be allowed, although "Alys's" is probably better, particularly as that is closer to how people would say it.
The Penguin Guide to Punctuation supports this:
'A name ending in s takes only an apostrophe if the possessive form is not pronounced with an extra s. Hence: Socrates’ philosophy, Ulysses’ companions, Saint Saens’ music, Aristophanes’ plays.'
Nevertheless your mileage may vary!
As a total aside; for some reason this amused me from Wikipedia as being a typically British example of pragmatism:
As the Palace of Westminster is still technically a Royal Palace, a convention has been adopted that any commoner who dies within the Palace is officially recorded as having died at St Thomas' Hospital to remove the need to convene a jury of members of the Royal Household under the Coroner of the Queen's Household.
For anyone (esp. Americans) who may be confused, the Houses of Parliament are housed within the Palace of Westminster.
Thanks. I hoped someone else might find it amusing. :-)
Annoyingly, it's changed the order of my replies around so they make less sense. That one was supposed to, and initially did, follow the one now below. Ho hum.
Yes, it seems rules have evolved since I was at school where I was taught that neither should an extra 's' be added in writing, nor should it be pronounced - except where it would cause confusion.
It seems modern practice/guidance is that an additional 's' should be added after the apostrophe if it would naturally be pronounced that way. Interestingly, there is a very prominent disagreement with this in large letters across the Thames from the Houses of Parliament; "St. Thomas' Hospital". :-)
Sorry, I just realised I effectively just repeated what you had already said.
I don't think Alys' bride is correct -- as far as I know, singular nouns (including proper nouns) ending in s take 's as usual to form the possessive, with the exception of a handful of "classical/historical" names such as Moses', Jesus'. Alys is not quite in that league :)
In our school we are taught that all names/nouns that end in "s" take only an apostrophe, as in with my name "Ellis'" without an extra "s".
It may be another difference with American usage. I have to agree with Ellis. I think it has become less rigid in British English so that "s's" is optional, but "s'" is/was the rule. It was also the rule in speech - that is more relaxed now.
I'm from the US and I would use "Alys' bride" instead of "Alys's bride" when writing