Several variants of Additional Member Systems have been proposed, but basically they are a combination of the First-Past-The-Post system and party list voting. The purpose is to retain the best features of First-Past-The-Post while introducing proportionality between parties through party list voting.

Each voter has two votes, one vote for a single MP via First-Past-The-Post, and one for a regional or national party list. Half the seats or more are allocated to the single-member constituencies and the rest to the party list. The percentage of votes obtained by the parties in the party list vote determines their overall number of representatives; the party lists are used to top up the First-Past-The-Post seats gained by the party to the required number. So if a party has won two seats in the constituencies but in proportion to its votes should have five, the first three candidates on its list are elected in addition.

in Germany and it was also chosen by New Zealand in a referendum in 1993, (although in New Zealand it is called Mixed Member Proportional Representation or MMP). The new Scottish Parliament and the Welsh Assembly were both elected by AMS in May 1999 and 2003 as was the London Assembly in May 2000.

• It results in broadly proportional representation along party lines while ensuring that there is a directly accountable MP for each constituency.
• It retains a number of single-member constituencies.
• It has produced strong and stable governments in Germany (but not single party governments)
• Each elector has at least one effective vote. Even of they see no chance of winning in the single member constituency, people can use their second vote for a party they support and still have a limited say through an additional member.
• The separation of the vote, allows the voter to make an expression of popular approval or disapproval which is not possible under First-Past-The-Post.  Because the first vote does not affect a party's total representation, a voter can use it to express personal support for a candidate without necessarily helping that candidate's party.
• AMS would give people the government they wanted, keeping the link between MPs and voters as well as giving some value to all votes, via the additional members.

• It combines many of the faults of First-Past-The-Post with many of the defects of the list systems of PR.
• Half of all MPs are not directly accountable to any voters, just to their party leadership, and have no constituency.
• It creates two types of MP, one with a constituency role and duties and one without such a base.
• To retain some constituency MPs, constituencies would have to increase in size.
• The parties would retain power over selecting candidates for constituency seats and would have complete control over choosing their Additional Members.
• Those who are under-represented today may not fare any better under AMS.
• In Germany a party can win more constituency seats than its total entitlement, and is allowed to retain its extra seat(s) and the total membership of the Bundestaag is increased by that number over the standard 656.

Like AMS, AV+ is a mixed system composed of two elements, a constituency element and a top-up. Voters would have two votes - one for a constituency MP and the other from a regional list.

The constituency MPs are elected by the Alternative Vote (AV).   The so-called 'top-up' MPs are elected on a corrective basis from open party lists.

The system is not currently used anywhere in the world.  It is the system proposed by the Independent Commission on the Voting System (chaired by Lord Jenkins of Hillhead) to be put to the electorate in a referendum as an alternative to First Past the Post for UK General Elections.

• In the constituencies, the winning candidate has the support of at least 50% of the voters.
• People can vote for the candidates of their choice without fear of wasting their votes.  A voter can vote for, say, the Green Party, knowing that if the Green Party candidate is not successful then their vote will transfer to their second preference.  Tactical voting is no longer needed.
• It is a broadly proportional system.
• Everyone will have an incentive to vote, because their vote will count.
• In the top-up section, voters will be able to choose the best candidate to represent their party.

• Constituencies will be slightly larger than at present.
• As with AMS, there will be two categories of MPs.

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