MPs are expected to vote next week on whether there should be a referendum on changing Britain's voting system.
How does the existing voting system work?
It is brutally simple. Candidates who get the most votes in individual constituencies are elected as MPs. The party with more MPs than all the other parties put together forms the government.
What's wrong with it?
Nothing, according to defenders of it. Critics say it is unfair because the number of seats a party has does not accurately reflect the share of votes it receives. For example, Labour won the 2005 general despite only getting 35% of all votes cast. The system is particularly tough on smaller parties. Despite getting 22% of the vote in 2005, 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 is the prime minister proposing?
He wants a referendum on changing to the Alternative Vote (AV) system.
What is that?
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 and the Liberal Democrats want a fully proportional system where the number of seats a party wins is more closely aligned with the number of votes they get.
Will Mr Brown's proposals affect the general election?
No. A referendum would be held by the autumn of 2011 if the idea is approved by MPs.
Haven't Labour promised electoral reform before?
Yes. They promised a referendum on it in their 1997 general election manifesto but the idea was kicked into the long grass by Tony Blair following his landslide victory, saying he wanted to keep the link between MPs and their constituencies. A 1998 review headed by Lib Dem peer Lord Jenkins, argued for a mixed system known as Alternative Vote Top-Up. This would see up to 85% of MPs elected under the alternative vote system but on a nationwide constituency basis. A second vote would be held for the remaining 15% of MPs. They would be elected from a series of county and city lists, taking into account votes cast and the number of MPs already elected in each area.
What is different now?
If MPs back it, the next government will be committed to holding a referendum in law.
What if the Conservatives win the general election?
They would almost certainly overturn the legislation and scrap the referendum. They believe the existing first-past-the-post system guarantees strong, stable government.
Why is Mr Brown doing this now?
Constitutional reform, including possible changes to the voting system, was one of Mr Brown's stated priorities when he came to power in 2007 and supporters say these proposals are merely the fulfilling of that long-standing interest. But opponents believe it has more to do with politics - proposing the changes to help position himself as the candidate of "change" because he knows David Cameron will oppose the plans. The Liberal Democrats have also questioned this "deathbed conversion". They have always made proportional representation a price of forming a coalition government in the event of no party getting an overall majority after the election and although they are lukewarm about AV, they still see it as a "step in the right direction".
Will MPs back his proposals?
Not necessarily. Some Labour backbenchers are against electoral reform as they believe it will cost the party seats, while others are against AV because it does not go far enough and is simply a ploy to embarrass the Conservatives at the general election.
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.
Are all the systems used proportionate?
No. 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.