The money supply curve is vertical because the central bank, not the market, decides how much money exists in the economy. Since the quantity of money is set by policy rather than influenced by interest rates, it stays at the same quantity regardless of whether interest rates are high or low. On a graph, that fixed quantity shows up as a straight vertical line.
How the Money Market Graph Works
In the standard money market model, the vertical axis shows the nominal interest rate and the horizontal axis shows the quantity of money. The downward-sloping demand curve represents how much money people and businesses want to hold at various interest rates: when rates are low, holding cash costs less in missed earnings, so people want more of it. When rates are high, they’d rather park their money in interest-bearing assets, so the quantity demanded drops.
The supply curve, by contrast, is a vertical line at whatever quantity the central bank has chosen. The interest rate where these two curves intersect is the equilibrium rate, the rate that makes people willing to hold exactly the amount of money that actually exists.
Why the Quantity Doesn’t Respond to Interest Rates
Most supply curves in economics slope upward. When prices rise, producers make more. Money doesn’t work that way. Commercial banks and private lenders don’t simply print more currency when interest rates climb. The total money supply is determined by the central bank’s decisions, not by what the market is willing to pay for money.
Economists call this an “exogenous” variable, meaning it’s set from outside the model. The central bank picks a target, uses its tools to hit that target, and the quantity of money stays at that level whether interest rates happen to be 2% or 8%. Because the quantity doesn’t change in response to the variable on the vertical axis (the interest rate), the curve is perfectly inelastic, which is just another way of saying it’s a straight vertical line.
How Central Banks Fix the Money Supply
The Federal Reserve, and central banks around the world, control the money supply primarily through open market operations: buying and selling government securities. When the Fed wants to increase the money supply, it buys securities from banks and pays with newly created reserves. The bank now has more money in its account at the Fed, which increases the monetary base. When the Fed wants to shrink the money supply, it sells securities, pulling reserves out of the banking system.
The St. Louis Fed puts it plainly: the Federal Reserve has “essentially complete control over the size of the monetary base.” This control is what makes the vertical line possible. The Fed decides the quantity, and the market determines the interest rate that balances supply and demand at that quantity. If demand for money increases (the demand curve shifts right), the interest rate rises, but the supply curve stays put because the Fed hasn’t changed its policy.
What Shifts the Curve Left or Right
Even though the money supply curve doesn’t slope, it does move. When the central bank changes policy, the entire vertical line shifts horizontally. If the Fed conducts open market purchases and injects more reserves into the system, the vertical line moves to the right, representing a larger money supply at every interest rate. This shift tends to push the equilibrium interest rate down. If the Fed sells securities and reduces reserves, the line shifts left, and interest rates tend to rise.
This is the core mechanism behind monetary policy in the traditional model. The central bank doesn’t directly set interest rates by decree. It moves the vertical supply curve, and the new intersection with money demand produces a different equilibrium rate.
The Monetarist Assumption Behind the Model
The vertical money supply curve comes from a school of thought sometimes called “verticalism.” Both early Keynesian economists and monetarists treated the money supply as something the central bank fully controls through the monetary base and the money multiplier. In this framework, the central bank controls reserves, reserves determine how much lending banks can do, and total lending determines the money supply. The chain runs in one direction: central bank decisions cause the money supply, not the other way around.
This is a simplification, and it’s worth knowing where it breaks down. Some economists argue the money supply is actually “endogenous,” meaning banks create money by making loans in response to demand, and the central bank accommodates afterward. In practice, modern central banks like the Fed often target an interest rate rather than a specific quantity of money. When the Fed announces a target range for the federal funds rate, it adjusts the supply of reserves as needed to keep rates near that target. In that scenario, the money supply responds to market conditions, which would make the curve less than perfectly vertical.
But in introductory and intermediate economics courses, the vertical supply curve remains the standard model because it captures an important truth: the quantity of money in an economy is fundamentally a policy choice, not a market outcome. Interest rates don’t cause the Fed to print more or less money. The Fed decides how much money to supply, and the market adjusts around that decision.