The 500km stretch from Dundalk to Derry becomes the only de facto land frontier between the EU and the UK after Brexit.
* The border-free EU single market ended customs controls in January 1993. Ceasefires leading to the 1998 Good Friday Agreement ended security checks.
* Brexit risks a return of customs controls, which could be a focus for violent attacks. The UK was originally determined to leave both the EU customs union and the single market. That would mean trade tariffs and product standard checks - a 'hard Border'.
* Brussels, London and Dublin agreed they did not want a hard Border. In December 2017, they agreed a formula now called 'the backstop' - in practice, the North stays in the customs union with no tariffs and would mimic EU single-market product standards, minimising checks.
* The DUP, propping the minority UK government, objected to the North getting different Brexit terms. The arrangement was extended to all the UK as a temporary insurance policy, expiring when a long-term EU-UK trade deal is done.
* Radical Brexiteers and pro-EU politicians objected, fearing it would leave the UK, in real terms, trapped inside the EU if they could not finalise a trade deal.
The EU has given assurances that this will not happen - but refused to fix a backstop end-date.