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