What is the additional member system?

The additional member system (AMS) is a mixed voting system where each voter has two votes. One is for a local constituency member and one is for a party in a larger region. Regional seats are shared out to make the overall result more proportional. It is used for the Scottish Parliament and the London Assembly.

The first vote elects a constituency member using first past the post. The second vote is for a party list in a region. Regional seats are then allocated using the D'Hondt formula, which takes into account how many constituency seats each party has already won. Parties that win few constituencies get more list seats. Voters can split their ticket, backing one party locally and another on the list.

The Scottish Parliament has 129 members. There are 73 constituency members and 56 regional members, seven from each of eight regions. The London Assembly has 14 constituency and 11 London-wide members. Wales used AMS for the Senedd until 2026, when it switched to a closed list system with 96 members. Germany and New Zealand use similar mixed systems for their national parliaments.

Supporters say AMS keeps a local representative while giving a much fairer overall result than first past the post. Critics point out it creates two types of member, and that list members are chosen by parties rather than directly by voters. It also makes majority governments less likely, which some see as a strength and others as a weakness.