The demand curve is one half of the familiar supply-and-demand graph that determines price and quantity exchanged in a perfectly competitive goods market. This curve shows the relationship between the price of a commodity and the quantity that buyers are willing to buy at each given price, holding all other factors constant. The shape of the curve, as well as how it shifts when various other factors do change, is explained by the theory of consumer behavior. The most important aspect of consumer theory is its conclusion that (in almost all circumstances) the demand curve for a commodity is downward sloping. This relationship is the familiar “law of demand” first articulated directly by Alfred Marshall (1842–1924) in his Principles of Economics (Marshall, ...