As MPs prepare to vote on Gordon Brown's plan to hold a referendum on the Alternative Vote (AV), here's a guide to how the various electoral systems would have changed the outcome in 2005.
The actual result. Labour won 356 seats, the Tories 198 and the Liberal Democrats 62. Labour's majority was 66.
Rerunning the 2005 election under the Alternative Vote reminds us that this method can produce even more distorted outcomes than first-past-the-post (FPTP). Under AV, Labour's majority rises to 88, the Liberal Democrats fail to clear the 100-seat barrier and minority parties make no gains.
The reason AV would swell the Labour majority (the party would have won 367 seats) is the high number of second-preference votes the party could expect to win from Lib Dem supporters. The Lib Dems would benefit in turn from second-preference Labour votes and would have won 74 seats under AV, 12 more than under first-past-the-post.
By contrast, the persistence of anti-Tory tactical voting in 2005 would have given the Conservatives just 175 seats, 23 fewer than they actually won. The Electoral Reform Society used opinion-poll findings from 2005 to estimate where voters' second preferences would have gone.
Alternative Vote Plus
Under the Alternative Vote Plus (AV+), which includes a second vote for a top-up list of MPs, we enter hung parliament territory for the first time. Labour's majority falls to 308, leaving it 15 seats short of a majority, and the Lib Dems make a major breakthrough, winning 110 seats. The Tories win 199, just one more than under first-past-the-post.
Single transferable vote
Under the single transferable vote (STV), Labour wins just 264 seats, leaving the party 61 seats short of a majority. The Lib Dems, who favour this system, pick up 147 seats, 73 more than under AV and 85 more than under first-past-the-post. The Tories win 200 seats, just two more than under FPTP.
Additional member system
If used in 2005, the additional member system would have produced the most proportionate outcome. Labour would have won 242 seats, 81 short of a majority, and the Tories 208 seats, ten more than under first-past-the-post. The Lib Dems would have picked up 144 seats, three fewer than under STV. But minority parties would have achieved their first breakthrough, with ten Ukip candidates, two BNP members and two Greens elected.