Recall from Section 1.1 that the demand function D(x) for a commodity relates the number of units x that are produced to the unit price p D(x) at which all x units are demanded (sold) in the marketplace. Similarly, the supply function S(x) gives the corresponding price p S(x) at which producers are willing to supply x units to the marketplace. Usually, as the price of a commodity increases, more units of the com- modity will be supplied and fewer will be demanded. Likewise, as the production level x increases, the supply price p S(x) also increases but the demand price p D(x) decreases. This means that a typical supply curve is rising, while a typical demand curve is falling, as indicated in Figure 1.36.