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, which do not interact with each other in any way; 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, then adding the two sets of results together.

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 combining a winner-take-all system with party-list proportional representation (PR). While first-preference plurality with PR is the most common pairing in parallel voting, 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. Parallel voting is used in both national parliaments and local governments in Taiwan, Lithuania, Russia, Argentina, and other countries, making it relatively common among the world's electoral systems.

In parallel voting, voters cast two (or more) votes, one for each method the system contains.^{[citation needed]} However, these votes do not interact in any way: the vote in one method has no effect on the calculation of seats in the other methods.

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. This sometimes leads to a hypercorrection that attempts to limit the term parallel voting to refer only to mixtures of first-past-the-post and proportional representation. Parallel voting can use other systems besides FPP, and can have any mixture of winner-take-all, semi-proportional, and proportional components.

Although the two are often mistakenly conflated, mixed-member majoritarian representation and parallel voting refer to two different things. Parallel voting refers to a rule for computing each party's representation in a legislature, which involves two voting systems operating in parallel, with one being layered (superimposed) on top of the other. By contrast, mixed-member majoritarian representation refers to the results of the system, i.e. the system retains the advantage that some parties get in the winner-take-all side of the system.

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 interaction (limited compensation) between the two components, for example this is the case in South Korea and Mexico. In South Korea, the hybrid of parallel voting and seat linkage compensation, being between the MMP and MMM type of representation has been called mixed-member semi-proportional representation as well.^{[citation needed]}