Additional-member system

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The additional-member system (AMS) is a two-vote seat-linkage-based mixed electoral system used for elections to the Scottish Parliament in the United Kingdom, although not for Westminster elections, 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. It is a form of mixed-member proportional representation and 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, which is a non-compensatory method. AMS is the name given to a particular system used in the United Kingdom that aims to provide proportional representation. However, in theory it can fail to be proportional. This is commonly caused by dis-proportional district results caused by seat overhang. The proportionality of AMS depends on having enough additional ("top-up") seats and on how votes are cast in a specific election. During its use in the United Kingdom, AMS 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 from 1999 until the 2026 election, the Senedd in Wales.