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 is used at the Scottish Parliament (Holyrood) with 73 constituency MSPs elected by FPTP and 56 regional-list MSPs, and was used at the Welsh Senedd from its founding in 1999 until 2021.

For 2026, Wales replaced AMS with a pure closed-list D'Hondt system across 16 six-member constituencies — the most significant Welsh electoral reform since devolution. The Scottish Parliament retained AMS for the 7 May 2026 Holyrood election. The two elections on the same night produced a direct methodological comparison: the Senedd D'Hondt result matched the YouGov final MRP projection on Plaid Cymru exactly (43 seats each), while the AMS Holyrood result showed a 7-seat SNP gap (projected 62, actual 58). The mixed-system Nature of AMS explains both results: constituency surprises such as Lorna Slater defeating Angus Robertson in Edinburgh Central produced list-correction effects that the D'Hondt projection models did not capture.

AMS and the 2026 Senedd's closed-list D'Hondt system are now in simultaneous live operation across two UK devolved legislatures, providing the clearest real-world comparison of the two main proportional methods used in British politics. AMS's list-correction mechanism gave Reform UK 17 Holyrood seats despite zero constituency wins — an outcome that pure D'Hondt might not have produced with the same constituency map, and that advocates of AMS argue reflects voter choice more faithfully than winner-take-all FPTP.