Surprisingly, history demonstrates that a monetary union is not necessarily predicated on the existence of a single currency. A monetary union could incorporate "several currencies, fully and permanently convertible into one another at irrevocably fixed exchange rates". This would be like having a single currency with various denominations, each printed by another member of the Union. What really matters are the economic inter-relationships and power plays among union members and between the union and other currency zones and currencies (as expressed through the exchange rate). Usually the single currency of the Union is convertible at given (though floating) exchange rates subject to a uniform exchange rate policy. This applies to all the territory of the single currency. It is intended to prevent arbitrage (buying the single currency in one place and selling it in another). Rampant arbitrage - ask anyone in Asia - often leads to the need to impose exchange controls, thus eliminating convertibility and inducing panic. Monetary unions in the past failed because they allowed variable exchange rates, (often depending on where - in which part of the monetary union - the conversion took place). A uniform exchange rate policy is only one of the concessions members of a monetary union must make. Joining always means giving up independent monetary policy and, with it, a sizeable slice of national sovereignty. Members relegate the regulation of their money supply, inflation, interest rates, and foreign exchange rates to a central monetary authority (e.g., the European Central Bank in the eurozone). The need for central monetary management arises because, in economic theory, a currency is never just a currency. It is thought of as a transmission mechanism of economic signals (information) and expectations (often through monetary policy and its outcomes). It is often argued that a single fiscal policy is not only unnecessary, but potentially harmful. A monetary union means the surrender of sovereign monetary policy instruments. It may be advisable to let the members of the union apply fiscal policy instruments autonomously in order to counter the business cycle, or cope with asymmetric shocks, goes the argument. As long as there is no implicit or explicit guarantee of the whole union for the indebtedness of its members - profligate individual states are likely to be punished by the market, discriminately. But, in a monetary union with mutual guarantees among the members (even if it is only implicit as is the case in the eurozone), fiscal profligacy, even of one or two large players, may force the central monetary authority to raise interest rates in order to pre-empt inflationary pressures.