Community hub
Recent from talks
Knowledge base stats:
Talk channels stats:
Members stats:
Additional-member system
The additional-member system (AMS) is a two-vote seat-linkage-based mixed electoral system used in the United Kingdom in which most representatives are elected in single-member districts (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 to the votes cast for party lists. It is a form of mixed-member proportional representation and 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 or top-up) – these are ignored under parallel voting, which is 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. This is commonly caused by dis-proportional district results caused by seat overhang. The proportionality of AMS depends on having enough additional ("top-up") seats and on how votes are cast in a specific election. During its use in the United Kingdom, AMS produced results closer to mixed-member proportional rather than mixed-member majoritarian representation.[citation needed]
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.
In an election using the additional member system, each voter casts two votes: a vote for a candidate standing in their local constituency (with or without an affiliated party), and a vote for a party list standing in a wider region made up of multiple constituencies (or a single nationwide constituency). In Scotland list members ("top-up" seats) are elected by region; in London there is a single London-wide pooling of list votes.
Voters are not required to vote for the same party in the constituency and regional votes. If a voter votes for different parties at the constituency and regional levels this is referred to as split-ticket voting. In the regional vote, the voter votes for a specific party, but has no control over which candidates from the party are elected. On the other hand, in the constituency vote, the voter votes for a specific candidate rather than a party.[citation needed]
The first vote is used to elect a member from their constituency under the "first-past-the-post" first-preference plurality (FPP) system (i.e. in the constituency, the candidate with the most votes takes the seat).
The second vote is used to determine how many additional seats a party may get, which is based on how many seats a party should get in total. Parties receive additional seats to match the vote shares they received as close as possible, making the legislature more representative of voters' preferences.
In the model of the AMS as used in the United Kingdom, the regional seats are divided using a D'Hondt method. However, the number of seats already won in the local constituencies is taken into account in the calculations for the list seats, and the first average taken in account for each party follows the number of FPTP seats won. For example, if a party won 5 constituency seats, then the first D'Hondt divisor taken for that party would be 6 (5 seats + 1), not 1. In South Korea, which uses the largest remainder method, constituency seats are taken into account by subtracting the number of constituency seats that the party won from the number of seats initially won by the party proportionally (over all seats).
Hub AI
Additional-member system AI simulator
(@Additional-member system_simulator)
Additional-member system
The additional-member system (AMS) is a two-vote seat-linkage-based mixed electoral system used in the United Kingdom in which most representatives are elected in single-member districts (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 to the votes cast for party lists. It is a form of mixed-member proportional representation and 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 or top-up) – these are ignored under parallel voting, which is 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. This is commonly caused by dis-proportional district results caused by seat overhang. The proportionality of AMS depends on having enough additional ("top-up") seats and on how votes are cast in a specific election. During its use in the United Kingdom, AMS produced results closer to mixed-member proportional rather than mixed-member majoritarian representation.[citation needed]
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.
In an election using the additional member system, each voter casts two votes: a vote for a candidate standing in their local constituency (with or without an affiliated party), and a vote for a party list standing in a wider region made up of multiple constituencies (or a single nationwide constituency). In Scotland list members ("top-up" seats) are elected by region; in London there is a single London-wide pooling of list votes.
Voters are not required to vote for the same party in the constituency and regional votes. If a voter votes for different parties at the constituency and regional levels this is referred to as split-ticket voting. In the regional vote, the voter votes for a specific party, but has no control over which candidates from the party are elected. On the other hand, in the constituency vote, the voter votes for a specific candidate rather than a party.[citation needed]
The first vote is used to elect a member from their constituency under the "first-past-the-post" first-preference plurality (FPP) system (i.e. in the constituency, the candidate with the most votes takes the seat).
The second vote is used to determine how many additional seats a party may get, which is based on how many seats a party should get in total. Parties receive additional seats to match the vote shares they received as close as possible, making the legislature more representative of voters' preferences.
In the model of the AMS as used in the United Kingdom, the regional seats are divided using a D'Hondt method. However, the number of seats already won in the local constituencies is taken into account in the calculations for the list seats, and the first average taken in account for each party follows the number of FPTP seats won. For example, if a party won 5 constituency seats, then the first D'Hondt divisor taken for that party would be 6 (5 seats + 1), not 1. In South Korea, which uses the largest remainder method, constituency seats are taken into account by subtracting the number of constituency seats that the party won from the number of seats initially won by the party proportionally (over all seats).