The UK is holding its second general election since 2015 - and third since 2010. Like the previous poll in 2017, it was called despite a law designed to ensure that elections only take place every five years. The so-called Fixed-term Parliaments Act has get-out clauses which parliamentarians clearly know how to use.
The reasons for holding this particular "snap election" are open to dispute. The governing Conservatives argued that it was needed because parliament is blocking Brexit. It's certainly true that the House of Commons has been unable to find a solid majority for any Brexit option, but whether you think that amounts to trying to "block Brexit" is a more complicated matter.
Some opposition members argued that because the Conservatives were losing votes it was clear that they could no longer govern. It was certainly true that the Conservatives no longer had a majority in the House of Commons, as a series of defections, rebellions and ejections meant the governing party could not guarantee the votes to get its business through parliament.
Clearly the declared reasons for supporting an election may not be the same as the actual reasons (which relate to party interests as much as anything) but enough political parties were keen on the December poll for parliament to vote for it.
To form a government, a party generally has to have an absolute majority of MPs. In 2010 no single party could manage this, so a coalition was formed between the Conservatives and the Liberal Democrats. In 2017 a looser arrangement was struck between the Conservatives and Northern Ireland's Democratic Unionists (DUP). It is possible for a party to run a minority government, but this is fraught with difficulties when it comes time to pass legislation through parliament.
Key characters in 2019
The Conservatives, led by Boris Johnson, went into this election as a minority government with some support from the DUP. The official opposition, the largest non-government party, was the Labour Party, led by Jeremy Corbyn. Corbyn has fought an election as leader before (2017) but this is Johnson's first time as head of his party. Other parties represented in parliament before the election included the Liberal Democrats, the Green Party and the Independent Group for Change. The Brexit Party did not exist at the time of the last election.
Johnson has previously been a journalist for several outlets, including the right-wing broadsheet The Daily Telegraph, and has been elected mayor of London. He was foreign secretary in Theresa May's government and led the official campaign to leave the EU in 2016. He became prime minister in mid-2019 when May resigned and Conservative party members chose him to replace her as party leader.
Corbyn has been an MP since 1983 but before becoming leader had not held a government or shadow spokesperson role. He was known in fact for rebelling against his own party on numerous occasions. He became leader in 2015 and was re-elected, after a challenge, in 2016.
However, despite the focus on these two men, the prime minister is not directly elected in the UK - that job goes to the leader of the party which forms government after the election.
Counting votes and forming governments
In a general election, voters in the UK go to the polls to choose 650 members of parliament. Voting closes at 10pm GMT - and counts begin almost immediately in most places. While a few areas always declare early, the majority of the results start coming in from 2am.
It may be some time before the shape of the next UK government is clear. That depends on which party has secured the most seats, and each of those is called individually by the official in charge of the local count. There is, however, usually at least one exit poll, which projects seat numbers for each party. These results are broadcast immediately after polls close.
For general elections, the UK uses a plurality voting system, more popularly referred to as "first past the post". In each constituency, the candidate with the most votes becomes the area's member of parliament. It's a simple system but it can mean MPs getting elected with less than 30% of the share of the local vote.
As a result of the system, elections are generally focused on a number of so-called "marginal seats". These are constituencies in which the vote was close last time and so which could change hands. The closest margin of victory last time unfolded in Scotland's North East Fife constituency, which was won by just two votes.
This all tends to skew the election experience, with voters in marginal seats experiencing intense competition while those elsewhere may feel that not a lot is going on. Some people in safe seats end up feeling their vote is wasted if they don't support the winning candidate.
As well as UK-wide parties, there are also parties that only stand in particular constituencies. The Scottish Nationalist Party only stands in Scotland, for example and Welsh nationalist party Plaid Cymru only stands in Wales. Many parties only stand in Northern Ireland and some UK-wide parties don't stand candidates in Northern Ireland at all. Some, such as the Brexit Party or those forming electoral pacts with other parties, are only contesting certain seats as a tactical move.
While 650 candidates will be selected across England, Northern Ireland, Scotland and Wales, there won't necessarily be 650 MPs. This is because while Irish nationalist party Sinn Fein traditionally stands candidates in the campaign, they refuse to attend parliament even if they win seats. The party won seven seats in the last election.
The number of MPs that can vote on legislation is further reduced by the Speaker, a member of parliament who does not vote and effectively becomes a neutral member. The previous speaker John Bercow, was a Conservative. He stood down shortly before the election and is being replaced by Lindsay Hoyle, a Labour MP.
Author: Paula Keaveney - Senior Lecturer in Politics, Edge Hill University