East German Mark
West and East Germany each had their own currency. Supposedly a mark was a mark. When civilians visited East Berlin, they had to get a certain amount of money to enter. U.S. (and other Allied) military personnel, often went to .East Berlin on their days off, as it was allowed (had to wear a dress uniform without name tags). And they did not have to change a certain amount of money, as the military didn't recognize the German Democratic Republic in any way...  Consequently, G.I.s visiting East Berlin had to get their East marks in West Berlin, where they were absolutely worthless. Banks would sell them at a rate of 10-15 East marks to one Deutsche mark.