The LM curve, the equilibrium points in the market for money, shifts for two reasons: changes in money demand and changes in the money supply. If the money supply increases (decreases), ceteris paribus, the interest rate is lower (higher) at each level of Y, or in other words, the LM curve shifts right (left). That is because at any given level of output Y, more money (less money) means a lower (higher) interest rate. (Remember, the price level doesn’t change in this model.) To see this, look at Figure 22.1 "Effect of money on interest rates when output is constant".