The Federal Reserve System implements monetary policy largely by targeting the federal funds rate. This is the interest rate that banks charge each other for overnight loans of federal funds, which are the reserves held by banks at the Fed. This rate is actually determined by the market and is not explicitly mandated by the Fed. The Fed therefore tries to align the effective federal funds rate with the targeted rate by adding or subtracting from the money supply through open market operations. The Federal Reserve System usually adjusts the federal funds rate target by 0.25% or 0.50% at a time.