In this reading, we will explore a model of household behavior that yields the consumer demand curve. Demand, in economics, is the willingness and ability of consumers to purchase a given amount of a good or service at a given price. Supply is the willingness of sellers to offer a given quantity of a good or service for a given price. Later, study on the theory of the firm will yield the supply curve.

The demand and supply model is useful in explaining how price and quantity traded are determined and how external influences affect the values of those variables. Buyers’ behavior is captured in the demand function and its graphical equivalent, the demand curve. This curve shows both the highest price buyers are willing to pay for each quantity, and the highest quantity buyers are willing and able to purchase at each price. Sellers’ behavior is captured in the supply function and its graphical equivalent, the supply curve. This curve shows simultaneously the lowest price sellers are willing to accept for each quantity and the highest quantity sellers are willing to offer at each price.

If, at a given quantity, the highest price that buyers are willing to pay is equal to the lowest price that sellers are willing to accept, we say the market has reached its equilibrium quantity. Alternatively, when the quantity that buyers are willing and able to purchase at a given price is just equal to the quantity that sellers are willing to offer at that same price, we say the market has discovered the equilibrium price. So equilibrium price and quantity are achieved simultaneously, and as long as neither the supply curve nor the demand curve shifts, there is no tendency for either price or quantity to vary from their equilibrium values.