Additional-member system

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The additional-member system (AMS) is a two-vote seat-linkage-based mixed electoral system used in the United Kingdom in which 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 United Kingdom 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 United Kingdom 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 the AMS in the United Kingdom. The AMS is used to elect the Scottish Parliament (in a regionalized top-up system) and the London Assembly (with at-large top-up seats), and formerly the Welsh Senedd.^[4]