Additional-member system

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The additional-member system (AMS) or is a mixed electoral system under which most representatives are elected in single-member districts (SMDs), and a fixed number of other "additional members" are elected to make the seat distribution in the chamber more proportional to the way votes are cast for party lists.^[1][2][3] It is distinct from parallel voting (also known as the supplementary member system) in that the "additional member" seats are awarded to parties taking into account seats won in SMDs (referred to as compensation or "top-up"), which is not done under parallel voting (a non-compensatory method).

AMS is generally categorised as either a a proportional or semi-proportional electoral system,in practice, the way that proportionality of AMS largely depends on the number of additional ("top-up") seats and the votes cast in a specific election. Sometimes AMS are considered the same mixed-member proportional representation (MMP), however MMP is a term that is itself sometimes applied more broadly or strictly.

This article focuses primarily on semi-proportional implementations of AMS, like the ones used in the UK. AMS is used to elect the Scottish Parliament, the London Assembly and formerly the.Welsh Senedd.^[4] In Scotland list members ("top-up" seats) are elected by region; in London there is a single London-wide pooling of list votes.