The term “hard border” is used to describe what happens to the UK/EU border in the event of a “no-deal” Brexit. But it’s actually quite a vague concept.

Leaving the EU’s single market without a withdrawal agreement in place means that trade across the border reverts to international rules set by the World Trade Organisation.

EU politicians talk about a hard border as “protecting the integrity of the single market”. In other words, they don’t want the single market to be exploited by non-EU members. And that means people and goods would be subject to checks and regulations.

Under a no-deal Brexit, this “hard border” would extend all around the UK. But, in particular, people use it to refer to the UK’s only land border – between Northern Ireland and Ireland.

Many people are worried that a hard border here could spark violence in Northern Ireland and potentially go against the Good Friday Agreement.

These concerns are real and significant. The chief constable of the Police Service of Northern Ireland has warned: “If we get this wrong we could drift back to almost a paramilitary style of policing.”

But what do people actually mean by ‘hard border’?

Since the EU referendum in 2016 people have made all kinds of claims and suggestions about what a hard border could look like in practice.

At its most basic, the term “hard border” can simply describe the legal situation that will come into place the moment no-deal Brexit happens (midnight on October 31st).

But the term is often used to describe more than just the legal situation – referring also to practical and physical changes on the Irish border. But ideas and expectations about this also vary greatly.

At one end of the scale, you may get the impression from certain commentators that it could involve a wall or a fence along the border. Some appear to have drawn comparisons to the Berlin Wall, while demonstrators built a “mock wall” to protest against no-deal Brexit.

At the other end of the scale, there are claims that a “hard border” could mean “little more than traffic cameras”. They say physical infrastructure could be minimal – or avoided completely.

Others talk about a hard border as something between these two extremes. No giant wall or fence, but some element of stop-points or border checks nevertheless.

Because the term “hard border” has become so broad – encompassing so many potential concepts – its use is often vague and confusing. This has often led to assumptions about what politicians mean when they use it.

For instance, in January the Guardian said: “The chief spokesman for Jean-Claude Juncker the European commission president, told reporters in Brussels it was ‘pretty obvious’ border infrastructure would be necessary if the UK were to leave without an agreement.”

But, in fact, the spokesman does not appear to have mentioned “border infrastructure”. Instead he said: “…In a no-deal scenario in Ireland, I think it’s pretty obvious – you will have a hard border. Our commitment to the Good Friday Agreement and everything that we have been doing for years with our tools, instruments and programmes will have to take inevitably into account this fact.”

So it seems the Guardian assumed that “hard border” in this context was referring to “border infrastructure”.

On another occasion, the EU Parliament’s Brexit coordinator, Guy Verhofstadt, told the House of Commons that no-deal Brexit “automatically… creates a hard border that has to be controlled”. In this phrasing, a hard border does not seem to refer to the physical controls themselves – but merely a legal situation that would require physical controls. A subtle but potentially important distinction.

What has the UK government said?

When she was prime minister, Theresa May repeatedly ruled out “any physical infrastructure at the border, or any related checks and controls” – even in the case of no-deal Brexit. But she added: “If the EU forces Ireland to do it, that’s down to them. We chose to leave; we have a responsibility to help find a solution.”

This sentiment has been repeated by Boris Johnson, who said: “The UK will never ever institute checks at the border, and I hope our friends in the EU would say the same.”

The government’s official “Operation Yellowhammer” report (which sets out predictions for no-deal Brexit in a worst case scenario) was formally published in September after being leaked to the Sunday Times the previous month. This document confirms that the UK would aim to “avoid an immediate risk of a return to a hard border on the UK side”.

However, it says this policy is “likely to prove unsustainable”. In a worst case scenario, the government expects the border situation would “severely disrupt trade” because of new tariffs and regulations that would automatically apply (whether or not any infrastructure is built). According to the report, this could lead to disruption, job losses, protests, direct action and road blockages.

The Yellowhammer document says, there will be “significant pressure to agree new arrangements… within days or weeks.” Presumably, all parties would want such an agreement to avoid infrastructure, so we’re back at square one.

The Sunday Times has also noted: “An earlier version of the document, seen by this newspaper, claimed that the UK’s ‘unilateral policy seeks to avoid a hard border’. However, that commitment was removed in the [August version] of the document.”

What have Ireland and the EU said?

Regardless of what the UK does, this does not stop the Republic of Ireland or the EU imposing physical border controls on their own side.

The Irish leader, Leo Varadkar, previously appeared to rule out any physical infrastructure – even under no-deal Brexit. He said this “will just never happen, ever”.

And, in March, he said: “We have made no preparations for a hard border between Northern Ireland and Ireland, no preparations for physical infrastructure or checks or customs controls.”

However, this position now seems to have developed. Earlier this month, Mr Varadkar said there would, in fact, be checks on goods and live animals if there was no deal. “As far as possible, these will take place in the ports, airports and at business level,” he said. “But some may need to take place near the border.”

Today, Mr Varadkar’s office told FactCheck it would not build any physical infrastructure on the border – even if there is no deal.

The EU, for its part, has not been specific about what infrastructure or border stops (if any) it may ask Ireland to put in place. Instead, they pointed FactCheck to a European Commission document containing just two paragraphs about the Irish border situation.

It says that, under no-deal Brexit, all goods entering Ireland from the UK will be “subject to the relevant checks and controls to protect the safety and health of EU citizens, preserve the integrity of the internal market and enforce compliance with fiscal obligations (duties, indirect taxes).”

It does not specify how or where those checks would be done, but it said EU officials were working to avoid a “hard border”.

How about a ‘Smart Border’?

In January, the Guardian revealed that almost 1,000 officers from England and Scotland would be trained for potential deployment to Northern Ireland if there was no-deal. It’s possible that police may have two functions: to prevent protests and violence; and using domestic policing to monitor smuggling and border regulation.

Of course, domestic policing already exists in the areas concerned. What’s more, police on both sides of the border already deal with border control issues – like smuggling and immigration crimes.

That’s because, although there is no physical border, some fiscal and regulatory differences already exist between the two countries. And these can provide incentive for crime.

For instance, there are tax differences between Northern Ireland and the Republic of Ireland for fuel, alcohol and tobacco – and the police help to enforce this. Likewise, the current lack of physical border means we rely on regular policing and intelligence to enforce certain immigration cases, such as stopping people with UK deportation orders from entering the country via Ireland.

At the moment, these things are dealt with using a combination of methods, including domestic police, HMRC enforcement and business coordination. Enforcement officers have the right to stop vehicles and seize goods (although this Irish Revenue document outlines strict and sensitive rules about how this is done).

A smart border solution to no-deal Brexit would see an expansion of this approach to cover the new tariffs and regulatory issues. Advocates suggest other methods could also be used as well, such as either using GPS or traffic cameras to track lorries. However, others point to several potential problems with this solution.

Firstly, it may not be very effective. After all, even the current system – which enforces rules for only limited numbers of goods – is not always effective and smuggling is already a problem. So a massive and sudden expansion of this would – at the very least – be a huge challenge.

A smart border could also create costs for businesses who need to take goods across the border – many of which are small or medium sized companies that may struggle with the extra bureaucracy.

Finally, there is always a risk that heavy enforcement could cross the line into a more physical presence on the border and spark violence.

The verdict

Both the UK and Ireland have now ruled out building any physical infrastructure on the border – even if there’s no deal.

But there’s still a possibility that the EU could force Ireland to do this against its will. And, even if physical infrastructure is avoided in the immediate days after no-deal Brexit, the bigger question is whether this is sustainable in the long term.

There is also a grey area here. When the UK and Ireland pledge not to build physical infrastructure on the border, that does not appear to rule out checks and infrastructure half a mile down the road instead. Also, could smaller things still be installed? Police stands and traffic cameras could become targets.

Until it actually happens, we can’t say for certain what a “hard border” will look like, because neither the UK, Ireland or the EU have spelled out what they would do in very much detail.