[Grammar] Names ending with a an s should have no S after the ' if it's a possessive

[past frost]

Example

"Carlos's" should be "Carlos'"
"Thomas's" should be "Thomas'"

[grand quest]

This is actually a stylistic thing, and the 's is often recommended, especially if you would pronounce the second s while speaking--which is what I do with both Carlos and Thomas. (Test it by saying "Carlos's book" or "Thomas's dog" out loud)