The border between Northern Ireland and the Republic of Ireland would be the only land border between the UK and the European Union if there is a Leave vote. Mishal Husain sees how that is colouring the EU referendum debate in those areas.
On the road west out of Londonderry, you have to pay close attention if you don't want to miss exactly where the UK comes to an end and the Irish Republic begins.
This is an understated, almost invisible border, marked only by the appearance of road signs in kilometres rather than miles, in Irish as well as English - and by the lower fuel prices at the petrol stations.
That is what people here are now used to. You can live on one side of the border and work in another, or shop on one side or another, depending on the sterling-euro exchange rate.
But many people here can remember a very different time, when the same road out of Derry housed a large British military checkpoint.
Historian Dr Billy Kelly showed me all that is left of it: some concrete paving alongside the grass verge, as well as a memorial plaque set into a nearby wall that tells a chilling story.
It commemorates an atrocity that took place at the same spot in 1990, when the IRA forced Patrick "Patsy" Gillespie to drive a van packed with explosives towards the checkpoint. It exploded before he could get out, killing him and five British soldiers.
Dr Kelly lived in Derry through the Troubles and tells me of the apprehension he would experience passing through the checkpoint.
"It was never something that you liked doing, because if there was even the slightest thing wrong, you could be held for hours," he says.
When I asked him how he felt today, driving along the same road entirely unhindered, he had one word: "Thankful."
Peace has brought the greatest dividend, but local business owner Donal Doherty says the EU has also played a role in the ease of living and working in the region.
"Peace has got rid of all the army and the checkpoints, and as far as I am concerned Europe and the free movement of goods made a huge difference in terms of taking customs posts off the roads," he says.
His award-winning restaurant, Harry's, is 500 yards inside the Irish border and is popular with people in Derry, especially when the pound is strong against the euro.
"This village used to be a stop village because Customs made you stop here," he says. "Now it's a place where you can trade across the whole of the north-west."
'Trade to continue'
The Vote Leave campaign says none of that would change in the event of a UK exit from the EU.
"The border would operate essentially as it does now," Lee Reynolds, their regional director tells me, because the arrangements on which it is based - the Common Travel Area between the UK and the Republic of Ireland - would be in all parties' interests to maintain.
"We are convinced that the Common Travel Area can continue to exist, that people's lives can carry on normally, that people would continue to be able to travel across the border and trade across the border."
But that doesn't entirely take away the concerns of people like Kevin McCool of KMC Tyres, who runs a business on the UK side, set up by his father 20 years ago.
Not only does he buy tyres from the Republic of Ireland - and wonders how Brexit might affect their price - he also depends on Irish customers coming to him because it is logistically easy and financially worth their while.
"Even a five or 10-minute stop is going to put people off," says Mr McCool.
"There is a lot of people just over the border who do what we do, and at the moment people do come to us from over there.
"But if it got to the stage of saving five euros or spending half an hour at a checkpoint, they're not going to cross."
Those British citizens who live along the UK-Irish border area already think in two currencies, often making daily decisions about where to buy what.
Some, like Kevin McCool, now live on the Irish side and travel back and forth every day.
For them there is a very practical dimension to the choice on 23 June - they are weighing up not just the UK-wide arguments but the question of how their everyday lives might be affected by a vote to leave or remain.