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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. It 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 can not affect the firm. The defining characteristic of both duopolies and oligopolies is that decisions made by sellers are dependent on each other.
Like a market, a political system can be dominated by two groups, which exclude other parties or ideologies from participation. One party or the other tends to dominate government at any given time (the Majority party), while the other has only limited power (the Minority party). According to Duverger's law, this tends to be caused by a simple winner-take-all voting system without runoffs or ranked choices. The United States and many Latin American countries, such as Costa Rica, Guyana, and the Dominican Republic have two-party government systems.
A commonly cited example of a duopoly is that involving Visa and MasterCard, who between them control a large proportion of the electronic payment processing market. In 2000 they were the defendants in a U.S. Department of Justice antitrust lawsuit. An appeal was upheld in 2004.
Examples where two companies control a overwhelming proportion of a market are:
In Finland, the state-owned broadcasting company Yleisradio and the private broadcaster Mainos-TV had a legal duopoly (in the economists' sense of the word) from the 1950s to 1993. No other broadcasters were allowed. Mainos-TV operated by leasing air time from Yleisradio, broadcasting in reserved blocks between Yleisradio's own programming on its two channels. This was a unique phenomenon in the world. Between 1986 and 1992 there was an independent third channel but it was jointly owned by Yle and MTV; only in 1993 did MTV get its own channel.
Duopoly is also used in the United States broadcast television and radio industry to refer to a single company owning two outlets in the same city.
This usage is technically incompatible with the normal definition of the word and may lead to confusion, inasmuch as there are generally more than two owners of broadcast television stations in markets with broadcast duopolies. In Canada, this definition is therefore more commonly called a "twinstick".
The Google-Facebook duopoly's share of the digital ad market is predicted at 61% in 2021.