Mixed electoral systems attempt to combine the positive attributes of both plurality/majority (or other) and PR electoral systems. In a mixed system, there are two electoral systems using different formulae running alongside each other. The votes are cast by the same voters and contribute to the election of representatives under both systems. One of those systems is a plurality/majority system (or occasionally an ‘other’ system), usually a single-member district system, and the other a List PR system.

There are two forms of mixed system. When the results of the two types of election are linked, with seat allocations at the PR level being dependent on what happens in the plurality/majority (or other) district seats and compensating for any disproportionality that arises there, the system is called a Mixed Member Proportional (MMP) system. Where the two sets of elections are detached and distinct and are not dependent on each other for seat allocations, the system is called a Parallel system. While an MMP system generally results in proportional outcomes, a Parallel system is likely to give results the proportionality of which falls somewhere between that of a plurality/majority and that of a PR system.

Parallel and MMP systems have been widely adopted by new democracies in Africa and the former Soviet Union.