I agree with Judith that this is a debated topic. My family name ends in S, so it is a problem for me. One grammar writer's advice is: use the extra "' 's " if you would pronounce the word with that extra syllable, but don't use the extra " 's " if you would not pronounce the extra syllable. Example: Write "Thomas's" because in English "Thomas's" is a three-syllable word. But write "The cars' horns were loud" because we say " cars' " as one-syllable. We don't say "car-zes." This is different advice than Judith's "be consistent" advice. I'm not saying it's better advice, but just that at least one grammar writer gives this advice. In my personal opinion, the extra " 's " often looks ugly and awkward, so I tend to not use the extra " 's ". But, I admit, I'm not consistent.

I agree with Judith and John. The spelling should be in line with the pronunciation. I add an 's' if I would naturally pronounce the 's' as a separate syllable, and I don't add one if I wouldn't pronounce it as an extra syllable. I do my best to be consistent (but don't always succeed!). I would definitely add an 's' to 'Thomas'. We always pronounce the genitive form as 'Thomas+iz' (the same as Thomases) so I would write this as " Thomas's ". A clear case where you wouldn't add an an 's' after the apostrophe is if the final 's' of the name is silent. For example, the name 'Louis' is pronounced 'Looee', so you would form the genitive by simply adding an apostrophe (Louis'). This is because you'd pronounce it as 'Looeez' rather than 'Looeeziz'. Other cases are not so clear-cut, but Strunk's example of "Old Mrs Chambers' estate' in Judith's answer is an instance of where the extra syllable ( "Mrs 'Chambersiz' estate")would sound so awkward that we would tend not to add it. So, accordingly, no 's' would be added to the spelling.

This is a highly debated topic. :-) Excerpted from The Guide to Grammar and Writing: "Some writers will say that the -s after Charles' is not necessary and that adding only the apostrophe (Charles' car) will suffice to show possession. Consistency is the key here: if you choose not to add the -s after a noun that already ends in s, do so consistently throughout your text. William Strunk's Elements of Style recommends adding the 's. (In fact, oddly enough, it's Rule Number One in Strunk's "Elementary Rules of Usage.") You will find that some nouns, especially proper nouns, especially when there are other -s and -z sounds involved, turn into clumsy beasts when you add another s: "That's old Mrs. Chambers's estate." In that case, you're better off with "Mrs. Chambers' estate.""

Thomas' pet dog is the correct answer. Anytime a word ends in an "S", an apostrophe will go after the s and you do not add another s to the word.