How do you write the possessive form of a name ending in s?

[MyBabyIsNameless]

For example, if Thomas wanted a plaque on his bedroom door to indicate it was his room, would it be Thomas's or Thomas'?

Another English teacher here who would write Thomas's.

I suppose it does depend on whether the final ‘s’ is pronounced or not. ‘Louis’s’ is just wrong!

Thomas’s room

You can use either
However, I prefer apostrophe after s

This is quite useful www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv57.shtml

Either are correct I believe but I prefer Thomas's. Thomas' just doesn't look right.

I think if you say the ‘iz’ bit, you add ‘s.

Agree Karwoman.
Jesus's room and Archimedes' room. You wouldn't put Jane' room on a plaque so why put Thomas' room? Jane's room and Thomas's room follow the same rule and look right.

You would say Thomas's room - pronouncing both s's - so I don't see how Thomas's can really be wrong.

Thomas's as the other version means that it belongs the all the people called Thomas.

No it doesn’t!

Thomas's
Jesus' and Moses' (to avoid two consecutive 'zuz' sounds)

There's no point in arguing about the logic of it. There are a gazillion illogical things about the English language. Both versions are correct and it is therefore a matter of preference. Personally I prefer s' with no second s.

Oh no why did I do this to my child and choose a name ending in s! I've set him up for a lifetime of this lol.

Teacher here and we would teach Thomas’s because it is a singular noun.

I would only use an apostrophe with no s if it were a plural noun eg: There are 14 cats. This is the cats’ room.

English is a mind-boggling language!

I like this guidance from a book called Her Ladyship's Guide to the Queen's English, published a few years back:

'With short names ending in s or es, add ’s.

Miss Jones’s class
Thomas’s doubts

With longer names, particularly biblical or classical ones, adding ’s may make the word difficult to pronounce. If so, just add an apostrophe – St Barnabas’ church, Euripides’ plays – or rephrase: the church of St Barnabas, the plays of Euripides.

Arguments rage over whether to use Jesus’ or Jesus’s when referring, for example, to Jesus’s miracles. Her Ladyship believes that euphony – making a pleasant sound – is the deciding factor here: most people pronounce the word ‘Jesuses’, which is not remotely awkward and makes Jesus’s a perfectly acceptable possessive. On the other hand, Hades’s is clumsy and Hades’ is therefore to be preferred. Again, rephrasing as the miracles of -or the jaws of Hades avoids any possibility of error.'

Thomas looks odd now I've read it so many times on this thread. (I know it's not)

As far am I'm aware, both are acceptable.

I know both are right, but there's a haulage company called Thomas's - Everytime I see one of their wagons, the double S makes me inwardly cringe.

Therefore I decree it shall be Thomas' in my house.

I clearly remember getting told off for writing James's by one of my English teacher.
"You should know its James' no 's' at your age Notso, my year seven bottom set know how to use apostrophes"

🖕if your reading this Miss Edwards

Thomas'

It can be both. Only sensible advice is chose one for yourself and stick to it (and understand anyone who does it the other was isn't"wrong").

(I've seen style guides which have names ending in an s having just the apostrophe and everything else apostrophe s.)

Thomas's as the other version means that it belongs the all the people called Thomas
No. That would be Thomases's room, wouldn't it. Big room - there's a lot of Thomases about!

I think the whole 'it's a singular noun', is LadySpratt's husband confusing two different rules.

Thomas's. It isn't awkward to say like that, so you write it as it's said.

🖕if your reading this Miss Edwards

The your in that sentence would be enough to stick the finger at Miss Edwards without the need for the image.

As others have said, both are correct. I don't really like Thomas', it looks like any old Thomas rather than your specific Thomas.