Central banks usually use OMO as the primary means of implementing monetary policy. The usual aim of open market operations is—aside from supplying commercial banks with liquidity and sometimes taking surplus liquidity from commercial banks—to manipulate the short-term interest rate and the supply of base money in an economy, and thus indirectly control the total money supply, in effect expanding money or contracting the money supply. This involves meeting the demand of base money at the target interest rate by buying and selling government securities, or other financial instruments. Monetary targets, such as inflation, interest rates, or exchange rates, are used to guide this implementation.