The international monetary system describes the structure through which exchange rates are determined, trade and capital flows occur, and balance of payments adjustments are made. It includes fixed and floating exchange rate systems, as well as the roles of central banks and reasons for each type of system. Over time, the system has evolved from early forms of money and bartering to the gold standard, Bretton Woods system of fixed rates tied to the US dollar, and currently a mixed system with major currencies floating and others using fixed or managed rates.