Punctuation question: The apostrophe ‘s conundrum

Punctuation question: Achilles’ or Achilles’s?

Jacques’s question: Please help me with a punctuation question. I keep seeing Achilles’ Heel on the web. I’m convinced it must be Achilles’s heel because he was one person. I know that, in the case of my own name, Jacques, it’s always Jacques’s whatever… So, surely, Achilles’s heel should work the same?

Tiffany’s answer: Achilles – like Moses, Charles, etc. – ends in a Z sound. So no double s. Jacques ends in a S sound (even though one doesn’t always pronounce it) – like Harris, Francis, etc. Hence the double s. It’s not about the number of people in this case but the hard (z) or soft (s) sound.