When demand and supply curves (which will be covered in the next chapter) were developed by economists, the convention that the “cause” be on the horizontal axis had not yet been established. As a result, for demand and supply curves, the cause is on the vertical axis.

The cause is price. Draw the curve that describes the following: “As oil prices fall, consumers buy more oil.” Place the cause (“oil prices”) onto the vertical axis and the result (“quantity of oil bought”) onto the horizontal axis.