LONDON — (CNN) Polls have opened in the UK, 52 days after Prime Minister Theresa May called a snap general election.

More than 46 million people are eligible to vote in what is the fourth major UK poll in three years, following the 2014 Scottish independence referendum, the 2015 general election, and the 2016 Brexit vote (to say nothing of local elections in 2014, 2015 and 2017).

Polls close at 10 p.m. UK time (5 p.m. ET), with results expected to begin rolling in within an hour or so after voting finishes.

May called the election three years earlier than scheduled ahead of what are expected to be tough negotiations with the European Union over Britain’s exit from the bloc.

What happens?

Voters in all 650 constituencies across England, Wales, Scotland and Northern Ireland are choosing their local member of parliament (MP) from the more than 3,300 candidates running throughout the UK.

In the UK system, the Prime Minister is the leader of the party able to form a government, meaning they need the support of the majority of the House of Commons — where MPs sit — or 326 seats.

Only two people have a realistic chance of being the next Prime Minister: May, the incumbent, or Labour leader Jeremy Corbyn.

Before the election, May had a majority of 12 seats, won in 2015 under Conservative Party leader David Cameron, who stepped down after losing the Brexit vote.

If she retains or bolsters that majority, she will stay on as Prime Minister (though losing seats could make her position as Conservative Party leader unstable).

Failure by any party to win a majority will result in what is a called a hung parliament.

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Could we see another coalition?

The UK’s first past the post system — whereby the candidate in each constituency with the most votes, rather than an absolute majority, wins the seat — means that hung parliaments have traditionally been rare, there have only been two since 1970.

In the past the two largest parties — Labour and the Conservatives — dominated British politics.

However, in recent years, a breakdown in traditional party loyalties has made everything more unpredictable.

In 2010, the Conservatives fell 20 seats short of a majority and were forced into a governing coalition with the Liberal Democrats.

That decision was made after negotiations between Labour and the Lib Dems broke down, showing how difficult it can be for two parties to hash out the deal necessary to govern — something that could be even harder this year.

Of the seven major parties contesting the election, two — the Conservatives and UKIP — sit on the political right, the Liberal Democrats sit roughly in the center, and four — Labour, the Greens, Plaid Cymru, and the Scottish National Party (SNP) — are on the left.

Within the wider left wing grouping however, are two nationalist parties, representing Wales and Scotland respectively. That makes negotiations even more complicated — the SNP for example are seeking a second Scottish independence referendum, which all UK-wide parties oppose.

So if there isn’t a clear winner Friday morning, we could be in for a long round of negotiations.

What about Brexit?

While terror attacks in Manchester and London have dominated discussion in recent days, the UK’s decision to leave the European Union has loomed large over the election.

Both the Conservatives and Labour have committed to enacting Brexit, but each party has a different vision for how the divorce from the EU should happen.

A so-called “hard” Brexit would see the UK leave the EU’s single market, which guarantees the free movement of goods, services and people within the bloc.

A “soft” Brexit would still mean Britain leaves the EU, but the government would seek to retain access to the single market and allow some degree of free movement.

As well as their differences over how to implement Brexit, the two parties also have major disagreements on how to handle the rewriting of a great deal of British law currently set by EU regulations.

The-CNN-Wire ™ & © 2017 Cable News Network, Inc., a Time Warner Company. All rights reserved. (PHOTO: Stefan Rousseau/PA via AP)