Improbable Island can be accessed via various subdomains. The “subdomain” is what comes after the “//” and before the “improbableisland.com” in the url. You can simultaneously log in to different accounts on each of these subdomains. This can be useful if you want to, say, roleplay several characters at the same time using the same browser. Here's a list of the various subdomains that either currently or previously allowed you to do this trick: