The Marshallian Cross

This exhibit illustrates the basic Marshallian cross market model diagram. The vertical axis measures the price of the good. The horizontal axis measures the quantity of the good exchanged. The negatively-sloped demand curve, D, represents the law of demand. The positively-sloped supply curve, S, represents the law of supply.

The intersection of the demand curve and supply curve at price Po and quantity Qo is the market equilibrium. A shift in either curve, caused by changes in the demand or supply determinants, results in a new equilibrium at, in all likelihood, a new price and new quantity.

An interesting little twist to the Marshallian cross was added by Alfred Marshall. Conventional graphical analysis usually displays the independent variable (X) on the horizontal axis and the dependent variable (Y) on the vertical axis. This allows analytical types to manipulate the X-independent-variable and observe how the Y-dependent-variable changes. In the Marshallian cross, quantity would then be the X-independent variable and the price the Y-dependent variable.

However, this configuration contrasts with typical uses of the market. Economists generally view price as the independent variable and quantity as the dependent variable. That is, they are concerned with how the price affects the quantity. In this context, price would work better on the horizontal axis, with quantity on the vertical.