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UK general election: The numbers you need to know to understand Thursday's pivotal vote

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The mathematical composition of this room will decide the fate of the UK
The mathematical composition of this room will decide the fate of the UK   -  
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We've had the pitches, politics and policy promises, now it's time for the cold, hard maths of the UK's snap general election.

Before we dive into the numbers, you first need to understand how the UK's system works. Each constituency elects one MP to parliament using first-past-the-post or, in other words, whichever candidate manages to win the most votes.

So what are the key figures you need to look out for as the results come in?

The mathematical finishing line

This is the point at which a party achieves a majority and no other combination of parties can possibly work against them.

The UK has 650 constituencies, each given one seat in parliament. The finishing line is always set at half of the total number of seats plus one.

In this case, that's 326 seats. That’s the goal for all parties campaigning in this election. Any party securing that number is guaranteed to win every vote in the House of Commons (assuming all MPs toe the party line in parliamentary votes).

The artificial finishing line

This is where the mathematics becomes really important. While in theory 326 is the magic number for a majority, in practice it can be slightly fewer seats.

Here's why...

The speaker of the UK's parliament and his three deputies are MPs but do not take part in votes because they are expected to remain impartial.

That has the effect of reducing the number needed for a majority to 324.

Then there is the Sinn Fein MPs from Northern Ireland. They do not take the oath of allegiance, and, therefore, even though they’ve been elected, they never actually become MPs. At the last election, there were nine of them.

Their absence has the effect of reducing the winning post to around 320.

So that’s the figure the major parties will be aiming towards. Anything more than that number provides a more stable government. Anything fewer than that and we’re in minority government or coalition territory.

And there are essentially three scenarios that can then play out.

Conservatives win a majority

If Boris Johnson’s party can secure 320 or more MPs, they will look to govern on their own. Polling through the campaign suggests this is the most likely option.

The relative strength of his government will depend on the majority he actually receives. A majority of 30 or more seats will provide a strong government that should last a full term.

The last time the Conservatives were in that position was 1987, when Margaret Thatcher won a 107-seat majority.

A small majority of up to 20 seats will cause concern.

In 1992, the Conservatives beat expectations and secured a small majority. But a series of lost by-elections, meant the government’s majority had been completed eroded by the time the parliament ended in 1997.

Conservatives fall just short of a majority

They would then need to seek the support of another party, either in a full coalition or in a so-called ‘confidence and supply’ deal.

This happened in 2010 when a full coalition with the Liberal Democrats was formed, and again in 2017 when the Conservatives were propped up by the Democratic Unionist Party (DUP).

If the Conservatives fall short this time, it’s hard to see where their coalition partners would come from: the DUP are fiercely against the Brexit deal Boris Johnson has brought back from Brussels.

It is also worth noting that a party finishing just two or three seats short of the finishing line may choose to go into government as a minority, and without a coalition partner. It’s a dangerous gamble, but it would require every opposition MP to work together to bring down the government.

Labour attempt to form a government

Jeremy Corbyn is unlikely to win an outright majority, but could secure enough seats to try and form a coalition with the Scottish National Party.

He’d also try to bring on board some of the minor parties, such as the Greens, the Welsh Nationalists and maybe some Northern Irish parties.

But there’d be big prices to pay in terms of concessions. If this occurred, expect another election in 2020, in addition to referendums on Europe and Scottish Independence.

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