Additional-member system

This article is part of a series on
Politics of the United Kingdom
Constitution Magna Carta Bill of Rights Treaty of Union (Acts of Union) Parliamentary sovereignty Rule of law Separation of powers Other constitutional principles
The Crown The Monarch (list) King Charles III Heir apparent William, Prince of Wales Royal family Succession Prerogative Counsellors of State Republicanism in the United Kingdom
Executive Privy Council Charles III (King-in-Council) HM Government Cabinet (list) Starmer ministry (L) Prime Minister (list) Keir Starmer (L) Deputy Prime Minister Angela Rayner (L) Departments Ministers Agencies
Legislature UK Parliament (King-in-Parliament) King Charles III House of Lords Lord Speaker The Lord McFall of Alcluith House of Commons Speaker Sir Lindsay Hoyle Leader of the Opposition Kemi Badenoch (C) 59th Parliament Opposition MPs List of MPs
Judiciary Charles III (King-on-the-Bench) Supreme Court President The Lord Reed Deputy President The Lord Hodge Justices of the Supreme Court United Kingdom England and Wales Scotland Northern Ireland
Bank of England Governor Andrew Bailey Deputy governors Monetary Policy Committee Currencies Banknotes Coins
Elections and referendums UK Parliament constituencies (2023 Periodic Review of Westminster constituencies) Political parties Political Parties, Elections and Referendums Act 2000 Dissolution and Calling of Parliament Act 2022 Scottish Parliament constituencies and electoral regions Senedd constituencies and electoral regions UK general elections 1801 co-option 1802 1806 1807 1812 1818 1820 1826 1830 1831 1832–33 1835 1837 1841 1847 1852 1857 1859 1865 1868 1874 1880 1885 1886 1892 1895 1900 1906 1910 (Jan–Feb) 1910 (Dec) 1918 1922 1923 1924 1929 1931 1935 1945 1950 1951 1955 1959 1964 1966 1970 1974 (Feb) 1974 (Oct) 1979 1983 1987 1992 1997 2001 2005 2010 2015 2017 2019 2024 Endorsements 2010 2015 2017 2019 2024 European Parliament elections (1979–2019) 1979 1984 1989 1994 1999 2004 2009 2014 2019 Scottish Parliament elections 1999 2003 2007 2011 2016 2021 Next Northern Ireland Assembly elections 1998 2003 2007 2011 2016 2017 2022 Next Senedd elections 1999 2003 2007 2011 2016 2021 Next UK referendums 1975 2011 2016
Devolution Intergovernmental relations in the United Kingdom Legislative consent motions Prime Minister and Heads of Devolved Governments Council Council of the Nations and Regions England Governance of England English votes for English laws Greater London Authority 1998 referendum Greater London Authority Act 1999 (2007 Act) Mayor of London Sadiq Khan Mayoral cabinet London Assembly London Plan Combined authorities Metro mayors Northern Ireland Northern Ireland Act 1998 (2006 Act) First Minister Michelle O'Neill deputy First Minister Emma Little-Pengelly Executive Assembly Act of the Northern Ireland Assembly Law of Northern Ireland Scotland Scottish devolution 1997 referendum Scotland Act 1998 (2012 Act, 2016 Act) First Minister John Swinney Government Scottish Parliament Act of the Scottish Parliament Scottish statutory instrument Scots law Wales Welsh devolution 1997 referendum (2011 referendum on law-making powers) Government of Wales Act 1998 (2006 Act, Wales Act 2014, Wales Act 2017) First Minister Eluned Morgan Government Senedd Measure of the National Assembly for Wales (1999–2011) Act of Senedd Cymru Welsh law
Administration England History Subdivisions: Regions Ceremonial counties (list) Metropolitan and non-metropolitan counties Metropolitan county Non-metropolitan county Greater London Unitary authorities (list) Districts (list) Metropolitan Borough Non-metropolitan District London Borough Civil parishes (list) Historic counties Northern Ireland History Subdivisions: Counties Districts Scotland History Subdivisions: Sheriffdoms Council areas Community council areas Civil parishes Shires of Scotland Wales History Preserved counties Principal areas Communities (list) Historic counties
Crown Dependencies Bailiwick of Guernsey (Guernsey Alderney Sark) Bailiwick of Jersey Isle of Man British Islands Common Travel Area UK-CD Customs Union
Overseas Territories Akrotiri and Dhekelia Anguilla Bermuda British Antarctic Territory British Indian Ocean Territory British Virgin Islands Cayman Islands Falkland Islands Gibraltar Montserrat Pitcairn Islands Saint Helena, Ascension and Tristan da Cunha (Saint Helena Ascension Island Tristan da Cunha) South Georgia and the South Sandwich Islands Turks and Caicos Islands British Overseas Territories citizen Visa requirements Visa policies British Overseas Territories Act 2002 UK-OT JMC Chagos Archipelago sovereignty dispute Gibraltar sovereignty dispute Falkland Islands sovereignty dispute
Foreign relations Foreign, Commonwealth and Development Office Foreign policy (history) Diplomatic missions of / in the United Kingdom UK passport British National (Overseas) passport Visa requirements UK–CD Customs Union Visa policy Free trade agreements Australia Barbados Belgium Belize Brunei Canada China Hong Kong Cyprus Denmark France Germany India Israel Italy Japan Kenya Malaysia Malta Netherlands New Zealand Norway Russia Saudi Arabia Singapore Spain South Africa Palestine Pakistan Poland Portugal United States Zimbabwe Ireland politics of Northern Ireland Good Friday Agreement UK–Ireland border Northern Ireland Protocol British–Irish Council European Union (history) Brexit Post-Brexit United Kingdom relations with the European Union UK–EU withdrawal agreement The Commonwealth member states United Kingdom and the United Nations United Nations Security Council P5 NATO G20 G7
United Kingdom portal England Northern Ireland Scotland Wales Other countries
v t e

A joint Politics and Economics series
Social choice and electoral systems
Social choice Mechanism design Comparative politics Comparison List (By country)
Single-winner methods Single vote - plurality methods First preference plurality (FPP) Two-round (US: Jungle primary) Partisan primary Instant-runoff UK: Alternative vote (AV) US: Ranked-choice (RCV) Condorcet methods Condorcet-IRV Round-robin voting Minimax Schulze Ranked pairs Maximal lottery Positional voting Plurality (el. IRV) Borda count (el. Baldwin) Antiplurality (el. Coombs) Cardinal voting Score voting Approval voting Majority judgment STAR voting
Proportional representation Party-list Apportionment Highest averages Largest remainders National remnant Biproportional List type Closed list Open list Panachage List-free PR Localized list Quota-remainder methods Hare STV Schulze STV CPO-STV Quota Borda Approval-based committees Thiele's method Phragmen's method Expanding approvals rule Method of equal shares Fractional social choice Direct representation Interactive representation Liquid democracy Fractional approval voting Maximal lottery Random ballot Semi-proportional representation Cumulative SNTV Limited voting
Mixed systems By results of combination Mixed-member majoritarian Mixed-member proportional By mechanism of combination Non-compensatory Parallel (superposition) Coexistence Conditional Fusion (majority bonus) Compensatory Seat linkage system UK: 'AMS' NZ: 'MMP' Vote linkage system Negative vote transfer Mixed ballot Supermixed systems Dual-member proportional Rural–urban proportional Majority jackpot By ballot type Single vote Double simultaneous vote Dual-vote
Paradoxes and pathologies Spoiler effects Spoiler effect Cloning paradox Frustrated majorities paradox Center squeeze Pathological response Perverse response Best-is-worst paradox No-show paradox Multiple districts paradox Strategic voting Lesser evil voting Exaggeration Truncation Turkey-raising Paradoxes of majority rule Tyranny of the majority Discursive dilemma Conflicting majorities paradox
Social and collective choice Impossibility theorems Arrow's theorem Majority impossibility Moulin's impossibility theorem McKelvey–Schofield chaos theorem Gibbard's theorem Positive results Median voter theorem Condorcet's jury theorem May's theorem Condorcet dominance theorems Harsanyi's utilitarian theorem
Politics portal Economics portal Mathematics portal
v t e

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.^[1][2][3] It 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 (a non-compensatory method).

AMS is the name given to a particular system used in the United Kingdom that aims to provide mixed-member proportional representation (MMP), however, when failing to reach proportionality (as it cannot compensate for overhang seats) it is classified as semi-proportional electoral system. In practice, the proportionality of AMS depends on the number of additional ("top-up") seats and the votes cast in a specific election. In parts of the United Kingdom where it is or was used, it produced results closer to mixed-member proportional rather than mixed-member majoritarian representation.

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 formerly the Welsh Senedd.^[4]