Reserve currency was formerly considered a monetary unit which, in accordance with the circumstances of the issuing country, could be exchanged for gold at any time. As such, other states considered this currency as a safe investment asset and purchased it. At different times the GBP and USD were popular reserve currencies. The reserve currency of today is any currency with a stable value (without sharp fluctuations) which can be freely exchanged into monetary units of another country and aids the issuing state to service their debts.