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London: There are always surprises on polling day. But political scientists agree that by far the most likely result of the election will be that no single party wins a majority.
So what next?
The question has vexed British politicians, the public, political scientists and constitutional lawyers for weeks, if not months. The answer is to be found in ancient custom, new law, regal privilege and, in the end, desperate sleep-deprived haggling in rooms across Whitehall (and possibly Edinburgh, Cardiff and Belfast).
Professor Simon Hix, from the London School of Economics, said there are few rules and "conventions can be broken" in the push for power after the election.
Tories argue that "legitimacy" is conferred on the party with the most seats in the House of Commons. Labour claims it's all about who can command a majority.
While public sentiment sides with the former argument, the law is much closer to the latter.
"There is no formal rule that the largest party or the incumbent has the first go at trying to form a government," Professor Hix said. "In fact, we have had examples in the past, back to 1924 when Ramsay MacDonald became prime minister. Labour did not have as many seats in the House of Commons as the Conservatives, but he formed a Labour government."
If there is no clear election winner, David Cameron must stay PM until he chooses to resign. When he resigns, the Queen chooses as his successor the person who appears most able to corral a Commons majority.
If the Conservatives have ended up with fewer seats than Labour, there could be considerable pressure on Mr Cameron from within his own party to step down and make way for his successor - perhaps Boris Johnson, a bookies' favourite.
The Queen has the power to dismiss a PM and personally pick a successor - but this hasn't been done since 1834 (by William IV) and even then it was controversial. So the parties and bureaucrats - and the PM himself - must discreetly let the monarch know when it's time to make a pick, and who should be picked.
The all-important issue, Professor Hix says, and the aim of all the post-election backroom deals, is to muster the "confidence" of the House of Commons.
"The only convention that matters is that the prime minister must obtain and retain support of a majority of MPs in the House of Commons," he said.
Theoretically, in a 650-seat parliament, 326 delivers a majority. However as the Irish nationalist party Sinn Fein do not take their seats, 323 is a workable number. If the Liberal Democrats end up with the numbers to pick a government - as they did last time - Nick Clegg has said publicly that the party with the most MPs after the election deserves the "time and space" to try to form a government, that is to try to negotiate a coalition or at least an alliance that can win a majority of votes in the Commons.
In other words, if the Conservatives get more votes than Labour, Mr Clegg will be knocking at Mr Cameron's door the next day. But they may find that together they still lack sufficient numbers.
If the horse-trading drags on without a clear result, the critical moment is the Queen's Speech, on May 27. Usually, this is where the government presents its program for office amid pomp and ceremony. However it may turn into a test of Mr Cameron's power, as MPs vote to "beg leave to offer our humble thanks to Your Majesty".
If the government loses the vote, Mr Cameron would almost certainly resign, though he technically has a few weeks to try to rustle together more allies. The Queen would then invite Ed Miliband to try to form a government, and he would have to face his own confidence vote in the Commons - and have his own alliance in place to win it.
If he lost that decisively, or if Mr Cameron did not resign but cannot win the confidence of the House, then the country would go to a new election, within weeks.
