Worlds that aren't the Actual World

In this post i'm going to give a cursory introduction to possible worlds talk as it is relevant to my project, so that I have something to refer back to when relevant.

So there's this world. Take the world to be everything. That is, everything there is. Worlds aren't planets, they're whole realms of being. So everything that you think of as true is true of this world. Now take one of those truths...for instance, Joe Biden is vice president. Now, you might want to say that the proposition expressed by the sentence "Joe Biden is vice president" has interesting properties besides being true. It also has the property of being contingent. That is, it didn't have to be true, and that's an interesting difference between the proposition expressed by "Joe Biden is vice president" and "all triangles are trilateral".