The Irish border has emerged as a crucial sticking point in the way of Brexit negotiations moving on to phase two in December.
Here are answers to some of the key questions on what will become the UK's only land link with the EU.
After Brexit in just over a year's time it may become the point where customs checks and tariffs could be levied on goods passing back and forth.
It is one of three issues which have to be resolved, along with citizens' rights and the settlement of the UK's financial obligations, before the EU agrees to move on to post-Brexit trade talks.
There are almost 300 crossings between north and south along what was once a heavily militarised 310-mile frontier. It is now almost invisible. You can drive freely between the North and Ireland, hardly noticing when you leave one jurisdiction and enter the other, apart from the different road signs.
Brexit itself is not the sticking point, it is the type of Brexit the UK envisages. If the UK left the EU but remained in the single market and customs union the border problem would arguably not look so insurmountable. The issue has been complicated by the UK's insistence on leaving Europe's trading and free movement frameworks, as it means the border will become a barrier between two different regulatory and economic zones.
In the summer, the Government attempted to put some meat on the bones of its oft-quoted pledge about not returning to the "borders of the past". Its position paper on the border envisaged the retention of an invisible frontier with no physical infrastructure.
The document proposed a customs arrangement that would see 80% of businesses on the island of Ireland entirely exempt from any new tariffs post-Brexit. That exemption would apply to small and medium-sized enterprises involved in localised cross-border trade. In respect of larger companies engaged in international trade, the Government suggested they could adhere to any new customs regime by completing retrospective declarations either online or at their premises.
The plan however was derided by EU figures, who claimed having an open border between two different regulatory frameworks was not credible.
Brussels believes a "soft" border can only be achieved if either the whole of the UK, or even just the North, remains either within the single market and customs union, or some specially-tailored system that adheres to those regulations.
The Government insists the whole of the UK must leave the EU on the same terms and that to leave the North essentially half in and half out would undermine the constitutional integrity of the UK. The Tories' confidence-and-supply deal with the Democratic Unionists at Westminster makes the chances of a Government change of heart on this key issue even less likely. The DUP has made clear that the North must be treated like any other part of the UK and has accused nationalists, republicans and even the Irish government of pushing the border plan as a means to bolster the case for a united Ireland.
No, as with most things involving the North, there is a significant political dimension. A free flowing border has been an outworking of the peace process. It is also symbolic of the hugely improved cross-border relations in recent decades.
Any move to resurrect checkpoints, albeit for very different reasons from the Troubles, has the potential to unsettle that dynamic. Some have even claimed it could be the spark that reignites violence, although others have dismissed such predictions as dangerous scaremongering.