Additional Member System (AMS)

The Additional Member System (AMS) is a mixed-member proportional electoral system in which voters cast two votes: one for a constituency representative elected by first-past-the-post (FPTP), and one for a regional party list. Regional seats are then allocated to correct the disproportionality produced by the FPTP constituency results, making the overall outcome broadly proportional while preserving local constituency representation.

AMS was used for the Welsh Senedd from its creation in 1999 until the 2021 election — five Senedd elections in total. For 2026 Wales has REPLACED AMS with a pure closed-list D'Hondt system across 16 six-member constituencies, the most significant electoral reform since devolution. The Scottish Parliament retains AMS for the 7 May 2026 Holyrood election, with 73 constituency MSPs elected by FPTP and 56 regional-list MSPs. Reform UK's projected 14 Holyrood regional seats under the Electoral Calculus MRP are all list seats, with zero constituency wins — a direct product of AMS's capacity to reward concentrated regional support without requiring constituency victories.

AMS and Wales's new closed-list system will produce directly comparable results on the same night: 7 May 2026 is the only UK election in history where two devolved legislatures elect on the same day using different proportional systems. That comparison will be forensic evidence for advocates on all sides of the wider Westminster voting-reform debate.