The Irish back-stop is a massive stumbling block in the UK Parliament for the Brexit withdrawal agreement between Britain and the EU.
The problem is how to manage the land border between Ireland and Northern Ireland if both sides cannot agree a trade deal.
The UK and EU have signed up to maintaining an open border without any physical checks after Brexit, rather than a hard border with customs posts and passport checks that would slow trade and movement.
But that only applies until the end of the transition period of 21 months ending in December 2020.
If both sides have failed to come to a trade agreement by then, the withdrawal agreement calls for the backstop to come into play.
What’s the row about the back-stop about?
Britain and Ireland are currently both in the EU customs union and single market which allows free movement of goods and people without security and customs checks.
At the end of the transition period, it’s feasible the UK will be outside the single market and customs union, which will mean any tariffs or duties will be charged at different rates and the introduction of customs checks to catch smugglers evading the charges.
The back-stop keeps Northern Ireland in the single market and customs union while the rest of the UK is outside of the regime.
This draws a line between the UK and Northern Ireland down the Irish Sea.
Is the back-stop inevitable?
Brexiteers claim this takes away British sovereignty as the European courts will still have jurisdiction over any disputes and the EU will make decisions impacting Northern Ireland without any input from the UK.
At worst, say some, this could lead to the break-up of the UK if no trade agreement is reached.
However, the EU will not come to the table to talk about trade unless Britain agrees to the back-stop.
The British government fears the back-stop could threaten the constitution of the UK – especially as the back-strop has no time limit.
The back-stop is not inevitable, but only comes in to play if Britain and the EU fail to agree a trade deal by the end of December 2020.
This means politicians are arguing about something that might never happen.