Economics and praxeology textbooks and treatises generally indicate price on the vertical axis of a supply or demand schedule, and quantity on the horizontal axis. In order to avoid confusing students who may be familiar with such sources, we adhere to that tradition here. Algebra and calculus students, however, are generally taught to graph functions by placing x on the horizontal axis and f(x) on the vertical axis. Because supply and demand schedules plot quantity as a function of price, the convention used in economics is inconsistent with that of mathematicians. One consequence is that supply or demand may exhibit a vertical slope in some price ranges, whereas vertical slopes never appear in functions graphed according to mathematical conventions. On the other hand, supply and demand schedules will never include portions with flat horizontal slopes, since the quantity supplied or demanded is uniquely determined by the price.