Apostrophe rules can get complicated when we refer to possession by plurals or possession by entities with names that end in "s". Example: "These are our drivers' cars." Note the position of the apostrophe in the plural case. It's hanging out there by itself. But we also have instances like "This is Elias' car." This was the accepted way of indicating possessives with names ending in "s" for many years, but more recently, it's become acceptable to double up the "s" as in "This is Elias's car." The reason why the latter usage has won out is because it's spelled the way we typically pronounce a sentence like this. We say /eliases/ [ə'laiəsəs], not /elias/. So we're pronouncing that second "s". But it didn't always used to be this way. I can recall when I was in grade school that the teachers were quite strict about only the single "s" being pronounced in possessive instances with names that end in "s". Even back then, the second "s" was pronounced in common vernacular, so I always regarded it as a stupid rule. And I guess enough others did too, such that the language prescriptionists (grammarians, who do make the rules and who do judge) finally relented and gave way.