Chile refers to its electoral system for the National Congress (Congreso Nacional) as the binomial system, which is equivalent to the d’Hondt system of proportional representation with open lists applied within 2-member districts. Each party or coalition may run only up to two candidates within each constituency.
Voters cast a ballot for a single candidate with seats allocated based on the total votes won by a party or coalition’s two candidates. The small district magnitude results in the first and second place party or coalition each winning one seat unless the first-place party or coalition gains at least twice as many votes as the second-place party or coalition. If a party or coalition gains only one seat, then the candidate with more votes receives the seat.
The high barrier to winning a seat and the need to double the second-place party’s votes to gain both seats encourages parties to form alliances and forces negotiations among them about which party runs candidates in which constituencies.
The following table shows the results from District 1 in 2013. The Nueva Mayoría coalition came in first but its share of the vote was not double that of second-place Si Tú Quiere, Chile Cambia coalition, so it received only one seat. The seat went to the Partido Socialista de Chile as its candidate received more votes than the Partido por la Democracia candidate. The Partido Liberal de Chile gained the second seat, as the leading party in the second-place coalition.
The next table shows the 2013 results in District 6. In this constituency, Nueva Mayoría received more than twice as many votes as the second-place Si Tú Quiere, Chile Cambia coalition, so it gained both seats (i.e. seats were won by both the Partido Demócrata Cristiano and the Partido Radical Socialdemócrata).