Electoral reform: So, just what system would give a fair result?

Even if Labour gets fewer votes, it can still beat the Tories. John Rentoul weighs up the possible alternatives and finds they all have their good points – and bad

The Liberal Democrat surge in the opinion polls has had the unexpected effect of casting a floodlight on some of the hidden paradoxes of the British system for electing members of parliament.

It seems bizarre that Labour could come third in votes and yet win the most seats. But the Liberal Democrat surge is too evenly spread to achieve a breakthrough, while the Labour vote is distributed most efficiently. So Gordon Brown can beat David Cameron even if the Labour share of the vote is 5 percentage points lower than that of the Conservatives, because the outcome of the election depends on the size of the gap between the Tories and Labour.

Surely, the cry has gone up, there must be a better and fairer way to elect our governments than this? Mr Brown, cannily but a little opportunistically, has got there before you, proposing a double referendum on a change to the "alternative vote" system for electing the House of Commons, and a House of Lords elected by "an open-list proportional representation electoral system". That would take the number of different voting systems in this country to seven.

First Past the Post

A primitive system in which voters mark a cross by the name of a single candidate. Used to elect MPs in 18 days' time, along with local councillors in England and Wales.

Pro Utterly simple; strong connection between MP and locality.

Con Voting seems pointless in most constituencies, which are safe; national results do not appear to reflect aggregate choices, as this weekend's opinion polls show; MPs can be elected by a minority of voters, who cannot express preferences.

Supplementary Vote

Voters mark a second preference, also using a cross, in a second column on the ballot paper. Used to elect mayors, most importantly the Mayor of London (where there are more than two candidates).

Pro Allows voters to express limited preference.

Con Not very intuitive; unsatisfactory halfway house to allowing voters to express preferences among all candidates (see STV, AV).

Single Transferable Vote

A system of preferential voting by which voters use numbers to list candidates in order of preference to elect several members in multi-member constituencies. Used to elect the Northern Ireland Assembly, MEPs in Northern Ireland, and local councillors in Scotland and Northern Ireland.

Pro Achieves proportionality between votes and seats within large constituencies, and is most likely to produce a proportional result nationally; allows voters to express a full range of preferences, including between candidates of the same party.

Con Proportional systems tend to give disproportionate power to smaller parties that hold the balance of power; the connection between votes and seats is a mathematical equation that looks horrible; the link between members and localities is weaker.

Additional Member System

Voters have two votes, one for a constituency member, and one for a regional party list, both single Xs. Regional members are elected to "top up" party representation to make it match total votes as closely as possible, which is not that close in most cases. Used for the Scottish Parliament, Welsh Assembly and London Assembly. A variant was proposed by Roy Jenkins for the House of Commons in 1998, but Tony Blair ignored it.

Pro Achieves greater proportionality, depending on the ratio of additional members to directly elected ones; maintains the constituency link for most members.

Con Tends to ensure small parties in permanent power (see STV); the British AMS does not allow voters to express preferences, although it could; it creates two classes of member, though this hasn't been a problem in practice.

Closed Party List System

Voters vote with an X for a party rather than for an individual in large multi-member constituencies. The highest candidates on each party's list are elected in proportion to votes cast. Used for European elections in England, Scotland and Wales.

Pro Proportional.

Con Voters cannot express preferences between candidates.

Alternative Vote

Voters mark candidates in order of preference in single-member constituencies. System proposed by Labour for the House of Commons.

Pro Voters can express preferences, ensuring each MP is elected with at least 50 per cent of the votes; keeps the constituency link.

Con Would retain most of the distortions of the present system, although the Lib Dems would win more seats.

Open List System

Labour's manifesto gives no details about how its proposed system for electing the second chamber would work, but presumably it would allow voters to list candidates in order of preference either within or across parties.

Pro Would weaken the grip of party machines (the main criticism of CPL).

Con A proportional system might be seen as more legitimate than that used to elect the House of Commons.