International economic relations also vary, in large measure, on monetary issues.  You are unlikely to accept the Turkish Lire in payment for your wages in this country, easily because you can't simply use that money to buy anything.  You need U.S. dollars in payment for your services, because you can simply spend the dollar.  Countries act the same way you do.  There are currencies that virtually everybody accepts as payment, and those widely accepted currencies are known as hard currency.