| Quantity | one | two | few |
|---|---|---|---|
| Sellers | Monopoly | Duopoly | Oligopoly |
| Buyers | Monopsony | Duopsony | Oligopsony |
A duopoly (from Greek δύο, duo 'two'; and πωλεῖν, polein 'to sell') is a type of oligopoly where two firms have dominant or exclusive control over a market, and most (if not all) of the competition within that market occurs directly between them.
Duopoly is the most commonly studied form of oligopoly due to its simplicity. Duopolies sell to consumers in a competitive market where the choice of an individual consumer choice cannot affect the firm in a duopoly market, as the defining characteristic of duopolies is that decisions made by each seller are dependent on what the other competitor does. Duopolies can exist in various forms, such as Cournot, Bertrand, or Stackelberg competition. These models demonstrate how firms in a duopoly can compete on output or price, depending on the assumptions made about firm behavior and market conditions.
Similar features are discernible in national political systems of party duopoly.