The Alternative Vote system ensures that the winning candidate in a constituency has to gain 50% of the votes.
Voters put the candidates in order of preference.
If a candidate receives 50% of first preferences they are elected.
If no one has 50% then the bottom candidate is eliminated and their second preference votes are allocated to the other candidates.
If there is still no candidate with 50% the bottom candidate is eliminated and the next preferences of their voters (for some this may be their third preference) are reallocated and so on until a candidate receives 50%.
A modified version, the supplementary vote system, is used for the election of Mayors in England and the Police and Crime Commissioners.
Voters can only give a first and second preference and, if no candidate has 50% of first preferences, then only the top two candidates remain and the second preferences of all the other candidates are reallocated and the candidate who then has the most votes wins.
In the Police and Crime Commissioner elections this led to a number of Independent candidates winning as many Labour and Conservative voters put them as their second preference rather than voting for the rival party. Thus the system can lead to the candidate voters least dislike winning, rather than the one they most like.
In the Agreement, drawn up by the Liberal Democrat and Conservative parties when they went into Coalition in 2010, the Conservatives allowed the Liberal Democrats to promote a referendum on whether elections for the House of Commons should be by the Alternative Vote system instead of First Past the Post.
This system would probably have helped the Liberal Democrats as, where they were the main challengers in a seat, Labour voters with their candidate in third place would probably have put them as their second preference to stop a Conservative MP from being elected and many Conservative voters would do the same where Labour and the Liberal Democrats where the top two parties.
The AV Referendum in 2011, called for by the Liberal Democrats, was lost by 68% to 32%.
The Conservative campaigned for a no vote while Labour was divided, with the leadership supporting AV but many prominent figures campaigning against.
There is some irony in the Conservative opposition because, if AV had been in place for the 2015 general election, more UKIP voters would have given their second preference votes to the Conservatives than to the other parties thus helping Conservative MPs to be elected.
Australia uses the AV system for its House of Representatives and so do many countries for the election of their President.
France has a two ballot system so that all candidates can stand in a constituency in the first election, candidates who get 50% are elected and, where this has not been achieved by anyone, then candidates who get over 12.5% of the vote contest the constituency the following week.
The system tends to produce a left and a right candidate in the second election and the one with the highest vote wins.
Elections for the US President follow a majoritarian system, whereby if a candidate gets most votes in a state then he or she gets allocated all the positions that a state has in an electoral college, and this college decides who is President.
