The marginal revenue curve plots the additional revenue a firm earns from selling one more unit; for a monopolist it lies below the demand curve (twice as steep if demand is linear) because lowering price to sell more units lowers the price on every unit sold, and profit-maximizing output is where MR = MC.
The marginal revenue curve graphs marginal revenue (the change in total revenue from selling one more unit) at every quantity. For a perfectly competitive firm, MR is just a flat line at the market price. For a monopolist, the story changes. A monopoly faces the entire downward-sloping market demand curve, so to sell an extra unit it has to cut the price, and that lower price applies to all the units it was already selling. The extra unit brings in its price, but the firm also gives up a little revenue on every other unit. That's why the monopolist's MR curve sits below the demand curve at every quantity past the first unit.
There's a clean graphing shortcut you'll use constantly. If demand is linear, the MR curve starts at the same vertical intercept and falls twice as fast. So if demand is P = 200 - 4Q, then MR = 200 - 8Q. The CED makes this curve the engine of monopoly analysis (EK PRD-3.B.6). The firm produces where MR = MC, then charges the price the demand curve allows at that quantity. Because MR is below demand, that price ends up above marginal cost, which is the source of monopoly inefficiency and deadweight loss.
This term lives in Topic 4.2 (Monopolies) in Unit 4: Imperfect Competition, supporting learning objectives 4.2.A and 4.2.B. The MR curve is the single biggest graphing difference between perfect competition and monopoly. Everything the CED asks you to do with a monopoly graph, finding profit-maximizing quantity at MR = MC, reading price off the demand curve above it, shading profit, consumer surplus, and deadweight loss, depends on drawing MR correctly below demand. EK PRD-3.B.6 states it directly. The monopolist sets MR = MC and charges a price greater than marginal cost. If you draw MR on top of demand, every answer that follows is wrong. This curve also explains why monopoly markets produce inefficiently low output, which is the conceptual heart of 4.2.A.
Keep studying AP® Microeconomics Unit 4
Demand Curve (Units 1-2, 4)
The MR curve is derived directly from the demand curve, and the two work as a team on monopoly graphs. MR tells you the quantity (where MR = MC), then you trace straight up to the demand curve to find the price. Mixing up which curve gives the price is the classic monopoly graph error.
Marginal Revenue (MR) (Unit 4)
The curve is just the picture of this number at every quantity. MR also tracks elasticity along demand. MR is positive where demand is elastic, hits zero where revenue is maximized, and goes negative where demand is inelastic. That's why a monopolist never produces on the inelastic portion of demand.
Allocatively Efficient Quantity (Unit 4)
Allocative efficiency happens where P = MC, but the monopolist stops where MR = MC. Since MR sits below demand, the monopolist's quantity is smaller than the efficient one. The gap between those two quantities is exactly where the deadweight loss triangle goes.
Lump-Sum Tax (Unit 4)
A per-unit tax shifts MC up, so the MR = MC intersection moves and quantity falls. A lump-sum tax touches neither MR nor MC, so quantity and price don't change at all. Exam questions love testing whether you know which taxes move the MR = MC point and which only eat into profit.
Multiple-choice questions hit this curve from two angles. First, the conceptual why. A stem like "a monopolist's MR curve lies below its demand curve because..." wants you to say the firm must lower price on all units to sell more. Second, the math. Given a linear demand like P = 200 - 4Q, you should double the slope to get MR = 200 - 8Q, set it equal to MC to find profit-maximizing quantity, and compare that to the revenue-maximizing quantity where MR = 0. Per-unit taxes and subsidies show up too. A tax raises MC, sliding the MR = MC intersection to a lower quantity, while a subsidy does the reverse. On FRQs, the standard ask is to draw a correctly labeled monopoly graph with demand, MR, MC, and ATC, identify quantity at MR = MC, price on the demand curve, and shade profit or deadweight loss, which is exactly what LO 4.2.B's calculation skills target.
The demand curve tells you the price buyers will pay at each quantity. The MR curve tells you how much total revenue actually changes when you sell one more unit. For a monopolist these are different numbers because selling more requires a price cut on everything. The firm uses MR (with MC) to pick its quantity, but it uses demand to set its price. In perfect competition the two collapse into one flat line at the market price, which is why this distinction only matters once you hit Unit 4.
A monopolist's MR curve lies below its demand curve because selling one more unit requires lowering the price on every unit, not just the last one.
For linear demand, the MR curve has the same vertical intercept as demand but twice the slope, so P = 200 - 4Q gives MR = 200 - 8Q.
The profit-maximizing quantity is where MR = MC, but the price charged comes from the demand curve directly above that quantity (EK PRD-3.B.6).
Because MR is below demand, the monopoly price exceeds marginal cost, output is below the allocatively efficient quantity, and deadweight loss results.
MR equals zero at the revenue-maximizing quantity, so the revenue-maximizing and profit-maximizing quantities are different whenever MC is positive.
A per-unit tax shifts MC up and reduces the MR = MC quantity, while a lump-sum tax changes neither curve and leaves quantity unchanged.
It's the curve showing the extra revenue from selling one more unit at each quantity. For a monopolist it lies below the demand curve, and the firm maximizes profit by producing where this curve crosses marginal cost.
To sell an additional unit, the monopolist must lower its price, and that lower price applies to all units sold, not just the extra one. The lost revenue on existing units makes MR less than the price at every quantity past the first.
Only for linear demand curves, but that covers nearly every AP graph and calculation. If demand is P = 50 - Q, then MR = 50 - 2Q. Same intercept, double the slope.
No, and this is the most common monopoly graph mistake. You find quantity where MR = MC, then go straight up to the demand curve to find the price. The MR curve gives quantity, never price.
A perfectly competitive firm is a price taker, so its MR curve is a horizontal line equal to the market price (MR = P = demand). A monopolist faces downward-sloping demand, so its MR curve slopes down and sits below demand.
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