The currency used as a reference in an international transaction or when setting an exchange rate. The key currency used is usually issued by a stable, developed country such as the United States. Central banks also hold key currencies in reserve (reserve currency).   



|||As a monetary practice, countries with smaller or less-dominant economies sometimes align their exchange rates with the dominant trading partner. The central bank of some developing countries may fix the exchange rate to the key currency, which has the effect of limiting monetary policy flexibility but can also increase confidence in the country's economy.