Compensation (electoral systems) in the context of "Additional-member system"

A majority bonus system (MBS, also called a minority-friendly majoritarian system) is a mixed-member, partly-proportional electoral system that gives extra seats in a legislature to the party with a plurality or majority of seats. Typically, this is done with the aim of providing government stability, particularly in parliamentary systems.

The size of the majority bonus can vary substantially, is usually a fixed number of seats, and may be conditional on the number of votes for each party. However, a relatively small majority bonus (such as in the reinforced proportionality system of Greece) may not always guarantee that a single party can form a government. At the same time, as the majority bonus is allocated in a non-compensatory manner, if the majority bonus is as high as 50%, and the largest party which has 50% of the popular vote receives it, this party may win as many as 75% of all seats available. This differentiates it from the similar majority jackpot system.

Mixed-member proportional representation (MMP or MMPR) is a type of representation provided by some mixed electoral systems which combine local winner-take-all elections with a compensatory tier with party lists, in a way that produces proportional representation overall. Like proportional representation, MMP is not a single system, but a principle and goal of several similar systems. Some systems designed to achieve proportionality are still called mixed-member proportional, even if they generally fall short of full proportionality. In this case, they provide semi-proportional representation.

In typical MMP systems, voters get two votes: one to decide the representative for their single-seat constituency, and one for a political party, but some countries use single vote variants. Seats in the legislature are filled first by the successful constituency candidates, and second, by party candidates based on the percentage of nationwide or region-wide votes that each party received. The constituency representatives are usually elected using first-past-the-post voting (FPTP). The nationwide or regional party representatives are, in most jurisdictions, drawn from published party lists, similar to party-list proportional representation. To gain a nationwide representative, parties may be required to achieve a minimum number of constituency seats, a minimum percentage of the nationwide party vote, or both.

A mixed single vote (MSV) is a type of ballot in mixed-member electoral systems, where voters cast a single vote in an election, which is used both for electing a local candidate and as a vote for a party affiliated with that candidate according to the rules of the electoral system. Unlike most mixed proportional and mixed majoritarian systems (such as parallel voting) where voters cast two votes, split-ticket voting is not possible under MSV. This significantly reduces the possibility of manipulating compensatory mixed systems, at the price of reducing voter choice. An alternative based on the mixed single vote that still allows for indicating different preferences on different levels is the mixed ballot, which functions as a preferential (mixed) single vote.

With MSV, voters usually cast their single vote for a local candidate in a single-member district (SMD) and then all votes (or just the wasted votes, depending on the system) from this lower tier are added to distribute seats between upper tier candidates, typically national party lists. How proportional the outcome is depends on many factors including the vote transfer rules (which votes are recounted as party list votes), whether there is a seat linkage, and other parameters (e.g. the number of list seats) used in the system. MSV systems originate from Germany and variations are currently used in Bolivia, Lesotho and Hungary for local elections in larger municipalities.