When no political party wins enough seats to secure a majority in the House of Commons, the result is a hung parliament.
The 2017 general election exit poll, reality is unfolding along the same lines, suggest that no party is on course to secure a majority. With 650 MPs sitting in Westminster, a party needs to win at least 326 seats to be sure of a majority. The Conservatives are forecast to win 314 seats and Labour 266.
The UK’s ‘first-past-the-post’ electoral system means hung parliaments rarely happen in Britain. 1974 election and more recently 2010 were the two occasions when the country found itself in a similar predicament.
In the case of a hung parliament, the leader of the party with the most seats is given the opportunity to try to form a government. This can take two forms - one option is a formal coalition with other parties, in which the coalition partners share ministerial jobs and push through a shared agenda.
The other possibility is a more informal arrangement, known as ‘confidence and supply, in which the smaller parties agree to support the main legislation, such as a budget and ‘Queen’s Speech’, put forward by the largest party but do formally take part in government.
What happens next?
The Conservatives look certain to have won the most seats. As a result, and as the incumbent, Theresa May will now have the chance to try to form a government. She could attempt to scramble together a formal coalition of other parties to obtain a majority in the House of Commons. Alternatively, she may try to lead a minority government if she can convince other parties to back her in a vote of confidence.
Either way, according to the current exit poll, it is difficult to see how May will be able to remain as Conservative leader for long. She began the election campaign with a sizeable lead and was widely predicted to extend her majority. However, a lacklustre campaign and criticism of the Prime Minister now looks like it could have denied the Tories the majority that once looked so certain.
Should May prove unable to form a government that can pass key laws when Parliament meets on 13 June, Jeremy Corbyn, as leader of the second biggest party, would then be given the opportunity to govern.
He would most likely attempt to do so as leader of a minority government, with the Lib Dems, SNP and Greens backing him on issues, such as Brexit and the economy, where they have shared ground.
What happens if no party can form a viable government?
In that situation, there is likely to be a second election – possibly later this year. However, under the Fixed Term Parliaments Act, passed in 2011, that would require a two-thirds majority of MPs to vote for a second election.
Under the provisions of that act, if a government loses a vote of confidence then other parties get two weeks to try to form a government. Only if a second confidence vote is lost is an early election held.