– Britain is facing its most uncertain election since women got full voting rights in 1928. The parties that dominated politics over the past century, the Conservatives and Labour, are struggling to reach voters. The Liberal Democrats, who in 2010 entered government for the first time in almost a century, may be on the verge of a wipeout. And there are a lot of newish kids on the block.

At stake is Britain’s membership in the European Union and the future of its union with Scotland. As is the decisionmaking ability of a U.S. ally that’s a permanent member of the United Nations Security Council. The result may be up in the air, but there are some important things to know about the way things will develop between now and May 7:

• Polls aren’t an infallible guide to the result. Opinion polls give a prediction of the vote each party will get. The difficulty comes in translating those percentages into seats in Parliament. Britain elects 650 members of the House of Commons, with each seat going to the candidate with the most votes. In 2010, the Conservatives got 36 percent of the vote and 47 percent of seats. By contrast, Labour under Tony Blair took 35 percent of the vote in 2005 and garnered 55 percent of the seats.

• The biggest party doesn’t get first shot at forming a government. After the inconclusive election of February 1974, Conservative Prime Minister Edward Heath, with fewer seats than Labour, stuck it out in his Downing Street office until the start of the following week in a bid to form a coalition. In 2010, talks between the different parties carried on in parallel. There are no rules about how parties should seek a coalition agreement.

• Just because he’s lost, the premier doesn’t have to resign. In fact, he isn’t allowed to step down until a successor is in place. “There’s not a formal mechanism,” says Philip Norton, professor of politics at Hull University. “He really goes at the point where it becomes clear that the other parties’ negotiations have reached a stage where they have a majority.”

• About five members of Parliament won’t take their seats. Sinn Fein, the Irish republican party, refuses to recognize the right of a Parliament in London to rule Northern Ireland, and so its elected members — five at the moment — have never taken their seats there. This reduces the number of votes needed to get a majority in the Commons to 323 or so from the official number of 326. Sinn Fein has rejected newspaper reports this year that it might change its policy.

• By U.S. standards, British elections are pretty cheap. The 2012 U.S. presidential campaign cost $7 billion. British spending limits for this election are 19.5 million pounds ($30 million) per party. Even if all three major parties hit that limit, something that their current levels of fundraising make unlikely, the whole campaign will cost less than $100 million.