Another instrument of monetary policy adjustment historically employed by the Federal Reserve System was the fractional reserve requirement, also known as the required reserve ratio. The required reserve ratio sets the balance that the Federal Reserve System requires a depository institution to hold in the Federal Reserve Banks, which depository institutions trade in the federal funds market discussed above. The required reserve ratio is set by the board of governors of the Federal Reserve System. The reserve requirements have changed over time and some history of these changes is published by the Federal Reserve.