History and use of instant-runoff voting

Instant-runoff voting (IRV) is a voting method used in single-seat elections with more than two candidates. Instead of voting only for a single candidate, voters in IRV elections can rank the candidates in order of preference. Ballots are initially counted for each elector's top choice, losing candidates are eliminated, and ballots for losing candidates are redistributed until one candidate is the top remaining choice of a majority of the voters. When the field is reduced to two, it has become an "instant runoff" that allows a comparison of the top two candidates head-to-head.

IRV more commonly known outside the US as the alternative vote or preferential voting, was devised around 1870 by the US architect W. R. Ware. Today it is in use at a national level to elect the Australian House of Representatives, the National Parliament of Papua New Guinea, the President of Ireland and President of India. In Australia it is also used for elections to the legislative assemblies (lower houses) of all states and territories except Tasmania and the Australian Capital Territory, and for the Tasmanian Legislative Council (upper house).

IRV is also used in a number of municipal elections in Australia, the United States, United Kingdom and New Zealand. Because of its relationship to the Single Transferable Vote (STV) system, IRV is used for by-elections and elections with only a single winner (such as elections for the President of Ireland) in some jurisdictions that use STV for ordinary parliamentary elections, such as the Republic of Ireland and Scotland.

IRV is known by different names in the various countries in which it is used. It is also known as the 'Alternative Vote', 'Ranked Choice Voting', and 'Preferential Voting', although IRV is only one of many ranked (or preferential) voting systems.