Additional member system

The additional member system (AMS), also known as mixed-member proportional representation (MMP) outside the United Kingdom,[1][2][3][4] is a mixed electoral system with one tier of single-member district representatives, and another tier of "additional members" elected to make the overall election results more proportional.

The term additional member system, introduced by the Hansard Society, has been largely replaced in the literature by the term mixed member proportional coined by New Zealand's Royal Commission on the Electoral System (1984–1986).[5] This article focuses primarily on semi-proportional implementations of MMP designed to yield moderately proportional election results, similar to the mixed systems used in the UK and referred to locally as AMS.