Editing Additional-member system (section)

{{Short description|Electoral system used in the United Kingdom}}
{{Use dmy dates|date=December 2018}}
{{Politics of the United Kingdom}}
{{Electoral systems|expanded=Mixed systems}}

The '''additional-member system''' ('''AMS''') is a two-vote seat-linkage-based [[mixed electoral system]] used in the [[United Kingdom]] in which most [[legislator|representatives]] are elected in [[single-member district]]s (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 representation|proportional]] to the votes cast for [[Electoral list|party lists]].<ref name="Additional-member system: Politics">{{cite encyclopedia|title=Additional-member system: Politics|encyclopedia=[[Encyclopædia Britannica]]|url=http://www.britannica.com/topic/additional-member-system|access-date=24 March 2016}}</ref><ref name="Elections in Wales">{{cite web|title=Elections in Wales|url=http://blogs.cardiff.ac.uk/electionsinwales/2013/11/05/electoral-systems-in-wales-3-ams/|publisher=[[Cardiff University]]|access-date=25 March 2016|archive-date=30 March 2016|archive-url=https://web.archive.org/web/20160330140326/http://blogs.cardiff.ac.uk/electionsinwales/2013/11/05/electoral-systems-in-wales-3-ams/|url-status=dead}}</ref><ref name="Electoral Reform and Voting Systems">{{cite web|title=Electoral Reform and Voting Systems|url=http://www.politics.co.uk/reference/electoral-reform-and-voting-systems|publisher=Politics.co.uk|access-date=25 March 2016|archive-date=8 April 2020|archive-url=https://web.archive.org/web/20200408184539/http://www.politics.co.uk/reference/electoral-reform-and-voting-systems|url-status=dead}}</ref> 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 (electoral systems)|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 [[Overhang seat|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]].{{Cn|date=January 2025}} 

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.<ref>{{cite web |title=Historic Act strengthens democracy in Wales &#124; GOV.WALES |date=24 June 2024 |url=https://www.gov.wales/historic-act-strengthens-democracy-in-wales}}</ref>