Making Plurals into Possessives
It’s all about the esses. You’ll see this most with proper names. That is Tom Maxwell’s. Easy enough. But what if it belongs to the whole Maxwell family? That is the Maxwells’s. Nope. It makes sense, but no. The correct way to make a plural possessive is to simply add an apostrophe after the s. That is the Maxwells’. And that’s how it is pronounced, too.
If I were to guess, I’d say it just gets into too many esses otherwise. But whatever the reason, to make a plural possessive, just put an apostrophe at the end.