IF YOU’RE PLANNING on staying up next Thursday night and into Friday morning to follow the results of the UK election, expect to hear the word ‘marginal’ used a lot.

You’ll hear several variations on it: ‘marginal seat’, ‘marginal constituency’ and even the perennial favourite, ‘key marginal’.

With good reason too. ‘Marginals’ are a vital part of the UK electoral landscape, so let’s take a quick look at exactly what the term means and what to look out for. 

With just one MP per constituency and the person with the biggest vote taking that seat, most seats in the UK usually stay in the same party’s hands election after election.  

In fact, there are 101 seats across England and Wales which have not changed hands since the Second World War. 

Seats that are not expected to change hands in an election are called ‘safe’ seats.

They are called this because a party’s numbers are strong enough to suggest that they are not in danger of coming second, or worse, and losing the seat.

Seats which are not considered safe and are perhaps up for grabs are therefore considered ‘marginal’.

Often, this is based on the previous elections and how much a party won a seat by the last time out.

The number of votes by which a party won a seat in a constituency is referred to as its ‘majority’.

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For example, the safest seat in the UK in 2017 was in the constituency of Walton in Liverpool where the Labour candidate won by 32,551 votes. 

The safety of a seat is usually ranked in terms of the victor’s winning margin of votes over the second placed candidate as a % of all valid votes cast, rather than the absolute number they won by.

In the case of Walton, Labour’s win there was a majority of 77.1%. 

By contrast, the most marginal or closest seat in the last election was in North East Fife in Scotland, where the SNP candidate won by just two votes. A statistical majority of 0.0%.

The most marginal seat in England was in Kensington where Labour pulled off a shock win to take the seat by just 20 votes, a majority of 0.05%.

Those examples are on the super-close end of the spectrum though, and seats need not be that close to be considered marginal.

A swing of a couple of percentage points from one party to another could mean a couple of thousand votes going the opposite direction, meaning that seats with a majority in the low thousands definitely come into play.

In general, seats are considered marginal if the majority is under 10% – this usually means about 5,000 votes. 

To give an example, the Conservatives won a seat in Watford by 2,092 votes in 2017, meaning that a 1.8% swing to Labour this time around would give them the lead.

This means that Labour considers Watford to be a ‘target’ seat, ie, a seat it feels it could take from the Conservatives.  

Marginal seats in Eastern England. PA Graphics PA Graphics

Flipping constituencies from one party to another is absolutely crucial in the final shake up because, in addition to gaining a seat for your party, you’re denying the other party one too.  

That may seem obvious but it’s worth mentioning, such is its importance to the numbers in parliament. 

It’s why the BBC puts so much emphasis on its ‘swingometer’ come election night, because small changes in the vote from one party to the other can have far-reaching consequences in marginal seats. 

So how many marginal seats are there?

About a quarter of the 650 seats have a majority of less than 10%, meaning they are marginals. Within that there are 51 seats where the majority is less than 2%, sometimes referred to as ultra-marginal.

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For Labour, many of its target seats are in London where polls indicate it is consolidating its support base. 

There are as many as nine seats which could go to Labour if it performs well, including a few that would change hands on a very small swing. 

Boris Johnson’s seat in the outer London constituency of Uxbridge & Ruislip South has a majority of just 5,034, the smallest of any sitting prime minister for 100 years.

While Johnson actually losing the seat would be a shock, there is a  concerted effort by Labour to make that happen. 

On the opposite side, Labour faces a battle to hold the marginal seats it has in Leave-voting areas, particularly in the north of England and the Midlands. 

It’s given rise to the question of whether a so-called ‘red wall’ of loyal Labour areas could be breached, devastating the party’s overall chances.  

For example, the North-east of England is traditionally a Labour heartland but the party is on the defensive in some areas, with the Conservatives eyeing up to half a dozen gains.

The constituency of Bishop Auckland has been a Labour seat since 1918 but its MP won with a majority of just 502 there in 2017.

The area voted Leave in the Brexit referendum and will be seen as a barometer of how the wider election pans out. 

One of the marginal seats that may be at risk on a really bad night for Labour is the former seat of Tony Blair in Sedgefield which has a majority of 7.3%. Another is Hartlepool, where the majority is 9.2%.

Even if the seats don’t flip, it will be worth keeping an eye to see if there’s any swing to the Conservatives, bearing in mind that a small change could have bigger implications in some other tighter marginals. 

The seats in this region will be among the first to declare on election night. Newcastle was the first in 2017 – so if you’re watching on the night, keep an eye on that swing.