A duopoly is an oligopoly with exactly two firms, where each firm's profit depends on both its own choice and its rival's choice. On the AP Micro exam, duopolies are the standard two-player setup for payoff matrices, dominant strategies, and Nash equilibrium (Topic 4.5).
A duopoly is the simplest version of an oligopoly. Instead of "a few" firms, there are exactly two. Everything that defines an oligopoly under EK PRD-3.C.1 still applies. There are high barriers to entry, the market is inefficient, and the firms are interdependent, meaning each firm's payoff depends directly on what the other firm does.
That interdependence is the whole point. If Firm A cuts its price, Firm B feels it immediately and has to respond. Because there are only two players, you can capture the entire strategic situation in a single 2x2 payoff matrix. This is why almost every game theory question in AP Micro uses a duopoly. It's not that two-firm markets are especially common in real life. It's that two firms is the minimum number you need for a "game" (EK PRD-3.C.3), where payoffs depend on everyone's choices, not just your own.
Duopoly lives in Topic 4.5 (Oligopoly and Game Theory) in Unit 4: Imperfect Competition, and it supports all three learning objectives there. You define game theory terms using duopoly tables (AP Micro 4.5.A), you find dominant strategies and Nash equilibria in two-firm games (AP Micro 4.5.B), and you calculate how big an incentive would have to be to change a duopolist's dominant strategy (AP Micro 4.5.C). Duopolies also show the collusion incentive from EK PRD-3.C.2 in its purest form. Two firms can agree to act like one monopoly, but each one is tempted to cheat. That tension between cooperating and defecting is the prisoner's dilemma, and it's one of the most reliably tested ideas in the course.
Keep studying AP Microeconomics Unit 4
Oligopoly (Unit 4)
A duopoly IS an oligopoly, just with the firm count pinned at two. Every oligopoly trait (high barriers to entry, interdependence, inefficiency) applies, but two firms makes the strategy easy to draw in a payoff matrix, which is why the exam loves it.
Nash Equilibrium (Unit 4)
In a duopoly game, the Nash equilibrium is the cell where neither firm wants to switch strategies given what the other firm chose. Finding it in a 2x2 matrix is one of the most predictable skills on the exam, so practice circling best responses for each player.
Cartel (Unit 4)
Two duopolists who collude on price or output have formed a cartel and are jointly acting like a monopolist. The catch is that each firm earns more by secretly cheating, which is exactly why cartels in payoff matrices tend to collapse toward the non-cooperative Nash equilibrium.
Monopoly (Unit 4)
A successful colluding duopoly produces the monopoly outcome, with lower output and higher price than competition would deliver. Comparing the collusive (monopoly-like) payoff to the cheating payoff is how you explain why collusion is unstable.
Duopoly is the default setup for game theory questions on both multiple choice and FRQs. You'll typically see a 2x2 payoff matrix with two firms (often named Firm A and Firm B) choosing between two strategies like high price vs. low price or high output vs. low output. You need to read the matrix correctly, identify whether each firm has a dominant strategy, find the Nash equilibrium, and explain why collusion breaks down when one firm can profit by cheating. Practice questions also test the price-war logic, where one duopolist cutting prices forces the other to respond, and quantity-leadership setups where one firm moves first and the other reacts. The calculation skill in 4.5.C shows up as questions asking how large a payoff change (like a fine or a side payment) would be needed to flip a firm's dominant strategy.
Every duopoly is an oligopoly, but not every oligopoly is a duopoly. Oligopoly means a few interdependent firms with high barriers to entry; duopoly means exactly two. The AP exam uses duopolies because two players fit neatly into a payoff matrix, but the economics (interdependence, collusion incentives, inefficiency) is the same for three or four firms. If a question says "two firms," treat it with all your oligopoly tools.
A duopoly is an oligopoly with exactly two firms, so it keeps all the oligopoly traits: high barriers to entry, interdependence, and an inefficient outcome.
Interdependence means each firm's payoff depends on both its own strategy and its rival's strategy, which is the definition of a game in the CED.
Duopolies are the standard two-player setup for payoff matrix questions, where you find dominant strategies and the Nash equilibrium.
Duopolists have an incentive to collude and act like a single monopolist, but each firm earns even more by cheating on the agreement, so collusion is unstable.
If one duopolist lowers its price, the other firm is pressured to match it, which can trigger a price war that pushes both firms toward the competitive outcome.
Some exam questions ask you to calculate how big a payoff change would need to be to alter a firm's dominant strategy.
A duopoly is a market with exactly two firms, each of whose profits depend on the other's decisions. It's the two-firm version of an oligopoly and the standard setup for game theory questions in Topic 4.5.
Almost. A duopoly is a specific type of oligopoly with exactly two firms, while an oligopoly can have a few firms (two, three, four, etc.). All the oligopoly rules, like interdependence and collusion incentives, apply to duopolies.
Two firms that collude can restrict output and charge the monopoly price, splitting monopoly profits (EK PRD-3.C.2). But each firm earns even more by secretly undercutting the agreement, so in a one-shot game both firms cheat and end up at the Nash equilibrium with lower profits.
The other firm loses customers fast and typically matches the price cut. That tit-for-tat response can spiral into a price war, which is exactly the interdependence the exam wants you to recognize.
No. Like all oligopolies, a duopoly is an inefficient market structure (EK PRD-3.C.1). Price ends up above marginal cost, output is below the allocatively efficient level, and high barriers to entry keep new competitors from fixing it.