Additional-member system

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The additional-member system (AMS) or is a two vote seat-linkage-based mixed electoral system used in the United Kingdom where most representatives are elected in single-member districts (SMDs), and a fixed number of other "additional members" are elected from a closed list to make the seat distribution in the chamber more proportional to the votes cast for party lists.^[1][2][3] It is distinct from using parallel voting for the list seats (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") - these are ignored under parallel voting (a non-compensatory method).

AMS is the name given to a particular system used in the UK that aims to provide mixed-member proportional representation (MMP), however, when failing to reach proportionality (as it cannot compensate for overhang seats) it is classified as semi-proportional electoral system. In practice, the proportionality of AMS depends on the number of additional ("top-up") seats and the votes cast in a specific election. In parts of the UK where it is or was used, it produced results closer to mixed-member proportional rather than mixed-member majoritarian representation.

This article focuses on the use of AMS in the UK. AMS is used to elect the Scottish Parliament, the London Assembly and formerly the Welsh Senedd.^[4]