For firms with more market control, especially monopoly, the average revenue curve is negatively-sloped. Because a firm with market control is a price maker and faces a negatively-sloped demand curve, its marginal revenue curve is also negatively sloped and lies below its average revenue (and demand) curve.

Perfect Competition

Marginal Revenue Curve, Zucchini Style

Marginal revenue is commonly represented by a marginal revenue curve, such as the one labeled MR and displayed in the exhibit to the right. This particular marginal revenue curve is that for zucchini sales by Phil the zucchini grower, a presumed perfectly competitive firm.

The vertical axis measures marginal revenue and the horizontal axis measures the quantity of output (pounds of zucchinis). Although quantity on this particular graph stops at 10 pounds of zucchinis, the nature of perfect competition indicates it could easily go higher.

This curve indicates that if Phil sells the first pound of zucchinis (an increase in production from 0 to 1), then his extra revenue is $4. However, if he sells his tenth pound (an increase in production from 9 to 10), then he also receives $4 of extra revenue. Should he sell his hundredth pound (an increase in production from 99 to 100), then he moves well beyond the graph, but his marginal revenue remains at $4.

Because Phil is a perfectly competitive firm, his marginal revenue curve is also his demand curve and his average revenue curve. All three curves coincide for perfect competition.

Monopoly, Oligopoly, and Monopolistic Competition

Marginal Revenue Curve, Medicine Style

The marginal revenue curve (MR) for Feet-First Pharmaceutical is displayed in the exhibit to the right. Key to this curve is that Feet-First Pharmaceutical is a monopoly provider of Amblathan-Plus and thus faces a negatively-sloped demand curve. Larger quantities of output are only possible with lower prices.

The vertical axis measures marginal revenue and the horizontal axis measures the quantity of output (ounces of medicine). Although quantity on this particular graph stops at 12 ounces of medicine, it could go higher.

This exhibit displays both the marginal revenue curve (MR) and the average revenue curve (AR), which is also the demand curve. Both are negatively sloped, but the marginal revenue curve lies below the average revenue curve, which means that marginal revenue is less than average revenue (and price) for any given quantity.