In the divorce discussions between Britain and the European Union, one of the biggest sources of acrimony is the Irish border.
Both the EU and the UK have agreed that there will be no checks or infrastructure of any kind at the Irish land border, and both sides hope they can achieve that in a future agreement on a new trading relationship after Brexit.
But if there were to be a delay or a failure in reaching such an agreement, a backstop would kick in automatically. The trouble is, the two sides can’t agree on what the backstop text should say.
The BBC’s Rob Young has been to Westminster to speak to a key player who’s keeping UK Prime Minister Theresa May on her toes during the negotiations.