Additional-member system

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. 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 proportional representation. However, in theory it can fail to be proportional, a situation called a seat overhang. 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 has 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 up until the 2026 election, the Senedd in Wales.