The Interstellar Monetary Unit (IMU - known colloquially as the 'credit') has been around for several hundred years. In addition, some of the minor colony worlds operate their own currencies - where they have not been able to trade sufficiently to attract banking services from one of the Big Seven interstellar clearing banks. Some nations do have internal currencies based on specific worlds - usually for historical reasons. Most famous is Earth's unwillingness to completely give up its ancient currency, the Dollar, and fully integrate with the IMU for domestic trading - arguing that such an integration would be giving up an important area of sovereignty.