Parallel voting

"Supplementary-member system" redirects here. For other uses, see Additional-member system, Mixed-member proportional representation, mixed-member majoritarian representation, and Majority bonus system.

A diagram of a common mixed system using parallel voting. The local tier (here FPTP) and the list tier have no compensation.

In political science, parallel voting or superposition refers to the use of two or more electoral systems to elect different members of a legislature. More precisely, an electoral system is a superposition if it is a mixture of at least two tiers; one portion of a legislature is elected using one method, while another portion is elected using a different method, with all voters participating in both. Thus, the final results are produced by filling the seats using each system separately based on the votes, with the separate groups of elected members meeting together in one chamber.

A system is called fusion (not to be confused with electoral fusion) or majority bonus, if it is an independent mixture of two system without two tiers. Superposition (parallel voting) is also not the same as "coexistence", in which different districts in the same election use different systems. Superposition, fusion and coexistence are distinct from dependent mixed electoral systems like compensatory (corrective) and conditional systems.

Most often, parallel voting involves the use of two separate systems to elect two separate groups of members, such as a winner-take-all system with party-list proportional representation (PR). First-past-the-post voting alongside PR is a common pairing in parallel voting, but many other combinations are possible.

The proportion of list seats compared to total seats ranges widely; for example 30% in Taiwan, 37.5% in Japan and 68.7% in Armenia.

Under the most common form of parallel voting, a portion of seats in the legislature are filled by the single-member first-preference plurality method (FPP), while others are filled by proportional representation. Parallel voting can use other systems besides FPP, and can have any mixture of winner-take-all, semi-proportional, and proportional components. For this reason, parallel voting is not always mixed-member majoritarian. For example, parallel voting may use a two proportional systems like STV and list-PR and then it would not be mixed-member majoritarian, and a majority bonus system (which is not the same as parallel voting) may also be considered mixed majoritarian. In addition, some mixed-member majoritarian systems are not parallel, in that they allow for compensation between the two components, for example this is the case in South Korea and Mexico.