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:

• http://www.improbableisland.com/(At some point after 2017-05-30 and before 2018-03-14, www and non-www were changed to access the same instance.)
• http://improbableisland.com/(At some point after 2017-05-30 and before 2018-03-14, www and non-www were changed to access the same instance.)
• http://seasonone.improbableisland.com/(Stopped working at some point after 2010-11-21)
• http://seasontwo.improbableisland.com/(Stopped working at some point after 2010-11-21)
• http://mail.improbableisland.com/(Stopped working at some point, then resumed working 2017-05-30 during II's move to https)(I think somebody told me once that using the mail subdomain puts undue strain on the servers, so don't use this. Just use a different browser)
• http://labs.improbableisland.com(Stopped working sometime after after 2011-12-11. Used to redirect to http://www.improbableisland.com/home.php, but eventually stopped doing even that, possibly due to II's move to https.)