The US dollar is the world's reserve currency. Central banks around the world tend to hold the dollar as an integral part of their foreign currency reserves because of the long history of economy and political stability in the United States. The euro is a young currency as it only began trading after the turn of this century. The members of the union exchanged their marks, francs, lira, pesetas, drachmas, escudos, and other foreign exchange instruments for the European currency. The only member that retained its currency was the United Kingdom, which turned out to