Every open economy has 3 goals: stable exchange rate, monetary policy autonomy, and free financial flows. However, only two of these can be achieved at once. A monetary union like the EU allows for free financial flows and a stable exchange rate, however it does not allow for monetary policy autonomy. A floating exchange rate (like the US) allows for monetary policy autonomy and free financial flows, but does not allow for a stable exchange rate. Capital controls (i.e. the prevention of free financial flows: citizens from investing abroad, and foreigners from investing domestically) allow for a stable exchange rate and monetary policy autonomy.