Lib Dem leader Nick Clegg has always said electoral reform would be a key point of negotiation in a hung parliament. Now both Labour and the Conservatives have pledged some movement towards changing the voting system. Here is a guide to the issue:

What is the current system?

For Westminster elections, it's first-past-the-post - the candidate who gets the most votes in their constituency is elected as the MP. If one party gets an overall majority - more MPs than all the other parties put together - they form the government. If no party gets an overall majority it is called a hung parliament and two or more parties would be expected to work together to form a government. You can read more in our general election FAQs.

How different systems work

First-past-the-post

Single transferable vote

Alternative vote

Alternative vote plus

Simple proportional representation

First-past-the-post What happened in 2010 In the current system, people get a single vote for who they want to represent their constituency and whichever candidate gets the most votes wins.

UK use: Election to Westminster and local government in England and Wales.

Single transferable vote Applied to 2010 result Several constituencies are combined and voters rank the candidates. Members are elected once they pass a certain number of votes, known as a quota.

UK use: Used in Northern Ireland for elections to Assembly, European Parliament and local government. Also used for local elections in Scotland.

Alternative vote Applied to 2010 result Voters rank the candidates. If no candidate has 50% of first preferences then second preferences are counted and so on until someone has a majority.

UK use: By-elections to Northern Ireland Assembly.

Alternative vote plus Applied to 2010 result The same as AV to elect most of the Commons but with a second element - the "plus" part - which would be used to elect 100 MPs in a more directly proportional system.

AV+ has yet to be put into practice anywhere in the world.

Proportional representation Applied to 2010 result The crudest version of proportional representation would give all parties seats in parliament based directly on their share of the vote. In practice, countries which employ PR have thresholds in place to screen out the smallest parties.

Simple PR is not in use in the UK.

Source: Electoral Reform Society

Why do the Lib Dems want the voting system changed?

They see it as unfair, saying first-past-the-post discriminates against smaller parties. Despite getting 23% of the vote in the election, the Lib Dems only won 9% of seats. Reformers say too many votes are effectively wasted in safe seats where either Labour or Conservatives have large, in-built majorities, and this depresses turnout. Results, they say, increasingly hinge on the preferences of a small number of voters in a handful of swing constituencies which is undemocratic.

What do they propose?

The Lib Dems support the Single Transferable Vote (STV) system. They say this will give people "the choice between candidates as well as parties".

How does STV work?

Instead of marking an X against just one candidate, the voter puts a 1 for their first choice, a 2 for their second, and so on. If a voter's number one choice reaches or exceeds a certain number of votes (known as the quota), they are elected. If the winning candidate received more votes than the quota, then the excess votes are transferred proportionately to other candidates based on the choices of those who voted for the winner. If no candidate in the second round meets the quota, then the candidate with the least votes is eliminated. The second preference votes of those who voted for that candidate are transferred to the next round. These transfers and eliminations continue until the number of candidates meeting the vote threshold is the same as the number of seats to be filled.

What else is different in the STV system?

It would mean the end to the current single-member constituencies for Westminster elections (you can find yours by clicking here). STV is designed for large, regional multi-member constituencies. According to one study, rural constituencies would become much larger, while some cities could see constituencies with electorates of 350,000 people. Parties would be able to field more than one candidate in each constituency - meaning a voter could vote for both the candidate and the party they prefer.

So Labour don't want STV?

They may not want STV, but they are quite keen on a different system known as the Alternative Vote (AV). It was in Labour's 2010 manifesto, and the BBC understands from Lib Dem sources that as part of coalition negotiations, Labour is offering to bring in AV, followed by a referendum on proportional representation.

How is AV different?

AV sees voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected. If that doesn't happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates. This process continues until a winner emerges.

Why would that be seen as fairer?

Under the current system, many MPs are elected on a minority of the overall vote in their constituency. Under the AV system, MPs could not be elected without the backing of at least 50% of voters in a constituency. This would increase the legitimacy of MPs - seen as an important factor in the wake of the MP expenses scandal - and increase choice.

Is this proportional representation?

No. Parties could still form a government with less than 50% of first choice votes. Campaigners such as the Electoral Reform Society want a proportional system where the number of seats a party wins is more closely aligned with the number of votes they get.

Why are Labour now into electoral reform?

The party will say it has been committed to reform since the 1997 manifesto which pledged them to "recommend a proportional alternative to the first-past-the-post system". However, while alternatives to first-past-the-post have been introduced for elections to the devolved administrations, the European Parliament and London Assembly, changing the system for Westminster elections has yet to happen. It wasn't until after the MPs' expenses scandal that Gordon Brown put forward the case for change. The government tried to introduce legislation that would require a referendum to be held by October 2011 in the Constitutional Reform and Governance Bill. However, this was dropped from the bill during the pre-election rush to get stuff through Parliament because of Conservative opposition.

Is Labour's backing for AV enough to get the Lib Dems' support?

In some ways yes, in some ways no. The Lib Dems argue that AV is not proportional, and can actually produce less proportional results than the traditional first-past-the-post system. In an added twist, Lib Dem leader Nick Clegg said in an interview in March that they would support an enhanced version of the Alternative Vote - known as AV+.

What's AV+?

AV+ is the Alternative Vote as outlined above, but as well as voters ranking constituency candidates in order of preference, they would get a second vote at a regional level either for a party or for their favourite candidate from a list proposed by the parties. This would mean having a group of constituency MPs and a group of "party list" MPs. Some commentators think having two different kinds of MP is a disadvantage of this system. It is also not used anywhere in the UK at the moment. Its supporters say AV+ is more reflective of the proportions of votes cast, and retains the MP-constituency links of first-past-the-post.

What is the Conservatives' position?

The Conservatives have been hitherto opposed to proportional representation and before the election, David Cameron said: "It doesn't put power in the hands of the people, it puts power in the hands of politicians." He added that first-past-the-post "is a decisive way of changing our government." Post election, however, the party leadership made a dramatic about turn during coalition negotiations and offered a referendum on the Alternative Vote system.

How does the electoral system work in Scotland and Wales?

Voting for the Scottish Parliament and Welsh Assembly is done through what is known as an additional member system. Some representatives are elected via the traditional first past the post method but voters get to cast a second vote for "top-up" seats, allocated in proportion to the number of votes. These representatives are selected on a regional basis from lists of candidates drawn up by each party - with five regions in Wales and eight in Scotland.

What about Northern Ireland?

In Northern Ireland local and Assembly elections (and Scottish local elections), voting is done on a single transferable vote basis which sees more than one candidate elected from a single constituency. Voters number candidates in order of preference and all those passing a defined threshold - calculated by dividing the number of valid votes by the number of seats plus one - are elected. Their surplus votes are distributed to other candidates on the basis of other preferences with low-scoring candidates being progressively eliminated.

What about mayoral elections?

The Mayor of London and other UK mayors are elected through a system known as the supplementary vote. Voters choose their first and second preferences and a candidate can only be elected in the first round if they get 50% of the vote. If no-one achieves this, all but the top two candidates are eliminated and their second preferences redistributed to the candidates still in the race. The candidate with the most votes is then elected. This is only suitable for electing a single office holder or MP.

How do they do things in the European Elections?

Voters in the recent European elections used a party list system. The UK was divided into large constituencies and different parties put together lists of candidates for election, with their preferred choices at the top. Seats were allocated, on a top-down basis, in proportion to parties' share of the vote.

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