The Additional Member System (AMS) combines a high degree of proportionality with local constituency representation. AMS is a 2-tier system - the voter usually has 2 votes - the first is a FPTP vote for the local constituency MP, and the second vote for their party of choice using a closed party list system for “top-up” members. (Alternatively, the voter can vote once for a constituency candidate and that vote can be counted a second time to allocate regional top-ups for that candidate’s party).
AMS involves two "layers" of MP - the first being elected to represent a constituency, in the same way as First Past the Post, and the second elected to represent a wider area, such as a region. It is this regional tier of MPs that gives AMS its proportionality. Constituency MPs can be nominated by their local parties, as under FPTP. With a closed list system, regional MPs are chosen from ranked lists submitted by regional parties.
The constituency votes are counted first, and seats allocated just as they are under FPTP; ‘regional MPs’ are awarded as ‘additional’ or ‘top-up’ seats to compensate for the disproportionality of the FPTP election. So, if a party has 5 MPs from the constituency votes and its fair share is 8 MPs then 3 candidates from its list become MPs. This is either done in regions, as in Scotland, or countrywide, as in New Zealand.
Where is this system used? AMS is used in New Zealand (where it is known as Mixed Member Proportional, or MMP), in Germany (since post-war 1949 and designed by Britain), and in a number of other countries. Three of the UK's devolved assemblies (London, Wales until 2026 and the Scottish Parliament) are elected using the Additional Member system.
AMS systems are extremely flexible. In New Zealand and Germany, half the seats in parliament are awarded on a regional basis, and this produces a very proportional system. The systems used for the UK's devolved assemblies were designed with a smaller proportion of seats allocated on a regional basis (Scotland has a 57% constituency to 43% top-up ratio) - this gives these systems a more limited degree of proportionality, although they perform far better than FPTP.
To avoid extremist parties having regional MPs elected, it is possible to impose a threshold, so that parties need to win a certain percentage of the popular vote, or at least one constituency seat, in order to get any regional seats. Also often applied for list PR systems, with variable thresholds in some countries (eg. Czech Republic, Slovakia) to set a higher bar for multi-party alliances.
See below for more information on the Additional Member System, including an explanation of how the regional votes are counted.
More info:
> More detail on AMS
> Electoral Reform Society explanation
> BBC video
> Electoral Commission video
> AMS: Issues in implementation and worked examples (Prof Maria Iacovou)
