Personal names ending in “s” can make the possessive with an apostrophe without an extra ”s” . We can say either Charles ’s wife was at the party with him. (more common) or Charles ’ wife was at the party with him. (less common)
Commonly the possessive is made with apostrophe without an extra “s”. Socrates ’ speeches Hercules ’ labors Venus ’ beauty
The possessive is formed with ‘s (singular ) or es’ (plural): The boss ’ s office is full of work to do. The boss es ’ offices are on the third floor.
When there are more than one possessor that own the same belonging , we add ‘s after the last possessor: John, Lucas and Carol ’s mother is here to talk to you. (The woman has 3 children: John, Lucas and Carol and is their mother.)
If there are more than one possessor , but each one owns a different belonging , we add ‘s after each possessor: Robert ’s and Luca ’s cars are parked over there. (Robert has one car and Lucas has another)