There are two key things to watch in the results in this referendum.
The first is the overall UK margin of victory. As Scotland showed in 2014, even a 10 point margin (55-45) is not considered in some quarters to have put the matter to bed for a generation.
The second thing to watch is whether the UK votes as one.
It is almost inconceivable that all four countries will back Brexit, because polls show large majorities for Remain in Scotland and Northern Ireland, despite signs of recent moves towards Leave in both countries.
It is highly possible, perhaps likely, that England will vote to Leave the EU. In that case, two further possible scenarios then emerge.
First, England votes to Leave, but by a narrow margin and votes in the other three countries are enough to outweigh the slender English majority.
Second, England votes to Leave, but by a sufficiently large margin that it outweighs the margin in favour of Remain in Scotland, NI and Wales (the latter country is thought to lean to Remain, but is more Eurosceptic than Scotland and NI).
Here are some numbers that show how tomorrow morning could pan out.
Recent polls show support for Remain at around 62% in Scotland, 60% in Northern Ireland. It is hard to get isolated polls for Wales and England but they are much closer with England perhaps narrow Leave and Wales narrow Remain.
Let us imagine Remain get 62% support in Scotland, 60% in NI, 53% in Wales, and 49% in England.
Apply those figures to a 70% turnout across the UK and the results would be:
• ENGLAND (Electorate 38.956 million)
Leave majority 550,000
• SCOTLAND (Electorate 3.988 million)
Remain majority 670,000
• WALES (Electorate 2.270 m)
Remain majority 95,000
• NORTHERN IRELAND (Electorate 1.26 million)
Remain majority 176,000
• UNITED KINGDOM (Electorate 46.474 million)
Remain majority 391,000
Note how a clear 550,000 margin for Leave in England can be outweighed by 941,000 majority for Remain in the three other UK countries, translating into an overall majority of 391,000 for Remain across the UK.
In that scenario the will of English has been thwarted by the Celtic fringe, which might well trigger resentment and a rise of English nationalism.
On the figures above support for Remain can drop almost as low as 48% in England and still the largest country by far in the UK has its will thwarted.
When English backing for Leave is 52% or above, it can outweigh the rest of the UK.
In that scenario, the resentment shifts to the other three countries for having had their wish thwarted by the English. There will be a particular focus on how Scotland reacts.
If the UK-wide vote is incredibly close, NI votes could decide it. If the result is 50.25% to 49.75%, the UK wide majority would be perhaps 160,000 votes, which might be less than the NI majority.
Such margins happen. Quebec voted 50.5% to stay in Canada in 1995. Denmark voted 50.7% to reject Maastricht in 1992.
Incredibly, the 107 million votes in the 2000 US presidential election came down to a 537 vote Bush win in Florida.
If you are interested in what happens to the UK-EU, make sure you vote today – you might just decide it.