There are two main families of electoral systems in the world: proportional and winner-take-all. All single-winner systems are, by definition, winner-take-all. Multi-winner systems may be proportional or winner-take all.
Sometimes it makes sense to elect just one person. For example, a nation would only ever choose one president at a time. However, when electing a legislative body, there is a real decision to make between using single-winner and multi-winner districts. That choice has profound consequences.
The academic consensus is that multi-winner districts are associated with:
On the other hand, single-winner districts are associated with:
Common single-winner systems include:
Plurality: A system in which the candidate with the most votes wins without necessarily a majority of votes. It is the most common system used in nation-states descended from the British and French Empires, including the United States and Canada.
Two Round System: A system identical to the plurality system except that if no winner attains the majority of votes in the initial election a second "runoff" round of voting takes place between the two candidates who received the most votes in the initial round.
Single-winner Ranked Choice Voting: A system in which voters rank candidates in order of preference. A candidate who receives over 50% of the first preference votes will be declared the winner; if this does not occur, the ballot count simulates a series of runoff elections. The candidate with the fewest first-place votes is eliminated, and ballots cast for that candidate are "transferred" to second choices as indicated on voters' ballots. This process of transferring votes continues until one of the candidates has a majority.
Common multi-winner systems include:
Block voting: A system in which electors have as many votes as there are candidates to be elected. Counting is identical to a plurality system, with the candidates with the most votes winning the seats.
Single Voting: A multi-winner system in which electors have one vote. The candidates with the most votes win.
List Proportional Voting: A multi-winner system in which political parties nominate candidates and electors vote for their most preferred party (or candidate nominated by a party). The seats are allocated to each party in proportion to the share received in the national vote.
Another choice, in addition to the one between single- and multi- winner districts, is whether to elect legislators proportionally or using something called "winner-take-all".
In proportional representation, groups of winners are allocated in alignment with the proportion of the vote they receive. For example, in a five winner district, a political party that received 38% of the vote would elect two candidates and a party that received 62% of the vote would elect three. Naturally, then, only multi-winner districts can be proportional.
Winner-take-all, by contrast, operates on the principle that the candidate(s) with the most votes win. This means that some voters get representation and others do not. For example, in a five winner district using winner-take-all, all five seats could be won by one party with just over half of the vote. Indeed, this sort of outcome was quite common in early congressional elections.
There are a multitude of different proportional systems, including the Single Voting and List Proportional Voting, as well as:
Cumulative Voting: A method of election in which voters have a number of votes equal to the number of seats to be elected. Voters can assign as many of their votes to a particular candidate or candidates as they wish. In a three seat district, for example, a voter could give all three of their votes to one candidate, two votes to one candidate and one to another, or one vote to three different candidates.
Ranked Choice Voting in Multi-Winner Districts: A method of voting in which voters have one vote but are able to rank candidates in order of preference. Initially, every ballot counts as a vote for its highest ranked candidate. Those candidates who have enough votes to win are elected and the weakest performing candidates are eliminated. For instance, in a five-seat district, a candidate is elected if they receive more than 1/6 of all votes cast, as this threshold ensures that they will be one of the top five finishers. If not enough candidates as number of seats reach the threshold to win, then voters' second choices come into play.
Winner-take-all systems include all single-winner district systems and the block vote.
Mixed systems—which combine single-winner winner-take-all elements with multi-winner proportional elements—are increasingly popular. Many consider them to be "the best of both worlds" because they maintain the link between constituent and representative in single-winner districts, while embracing proportionality.
The two main types of mixed systems are:
Mixed Member Proportional: An electoral system in which each voter gets two votes: one for a candidate in a local constituency and another for party. A fraction of seats are elected using plurality and the remainder from list proportional systems. The list seats are allocated after the plurality seats in such a way as to achieve proportionality with the national party vote.
Parallel Systems: An electoral system in which each voter gets two votes: one for a candidate in a local constituency and another for party. A fraction of seats are elected using plurality and the remainder from list proportional systems. The list seats are allocated proportionality with the national party vote, but the legislature itself need not reflect the party vote across the nation.