Duverger's law in the context of "Multi-party system"

Parliamentary opposition is a form of political opposition to a designated government, particularly in a Westminster-based parliamentary system. This article uses the term government as it is used in Parliamentary systems, i.e. meaning the administration or the cabinet rather than the state. In some countries, the title of "Official Opposition" is conferred upon the largest political party sitting in opposition in the legislature, with said party's leader being accorded the title "Leader of the Opposition".

In first-past-the-post assemblies, where the tendency to gravitate into two major parties or party groupings operates strongly, government and opposition roles can go to the two main groupings serially in alternation.

Maurice Duverger (/ˈduːvərʒeɪ/ DOO-vər-zhay; French: [mɔʁis dyvɛʁʒe]; 5 June 1917 – 16 December 2014) was a French jurist, sociologist, political scientist and politician born in Angoulême, Charente. Starting his career as a jurist at the University of Bordeaux, Duverger became more and more involved in political science and in 1948 founded one of the first faculties for political science in Bordeaux, France. An emeritus professor of the Sorbonne and member of the FNSP, he has published many books and articles in international newspapers, such as Corriere della Sera and la Repubblica in Italy, El País in Spain, and especially Le Monde in France.

Duverger studied the evolution of political systems and the institutions that operate in diverse countries, showing a preference for empirical methods of investigation rather than philosophical reasoning. He devised a theory which became known as Duverger's law, which identifies a correlation between a first-past-the-post election system and the formation of a two-party system. While analysing the political system of France, he coined the term semi-presidential system.

A two-party system is a political party system in which two major political parties consistently dominate the political landscape. At any point in time, one of the two parties typically holds a majority in the legislature and is usually referred to as the majority or governing party while the other is the minority or opposition party. Around the world, the term is used to refer to one of two kinds of party systems. Both result from Duverger's law, which demonstrates that "winner-take-all" or "first-past-the-post" elections produce two dominant parties over time.

The first type of two-party system is an arrangement in which all (or nearly all) elected officials belong to one of two major parties. In such systems, minor or third parties rarely win any seats in the legislature. Such systems exist, for example, in the United States, the Bahamas, Jamaica, and Zimbabwe. In such systems, while chances for third-party candidates winning election to major national office are remote, it is possible for factions within the larger parties to exert influence on one or even both of the two major parties.

Third party, or minor party, is a term used in the United States' two-party system for political parties other than the Democratic and Republican parties. The Electoral College for presidential elections and the plurality voting system for most other elections have established a two-party system in American politics. Third parties are most often encountered in presidential elections and while third-party candidates rarely win elections, they can have an effect on them through vote splitting and other impacts.

With few exceptions, the U.S. system has two major parties which have won, on average, 98% of all state and federal seats. According to Duverger's law two main political parties emerge in political systems with plurality voting in single-member districts. In this case, votes for minor parties can potentially be regarded as splitting votes away from the most similar major party. Third party vote splitting exceeded a president's margin of victory in three elections: 1844, 2000, and 2016.