Since 2005, there have been no checkpoints in place between Northern Ireland and the Republic of Ireland.
The border had once been dotted with British military checkpoints, and their removal was a landmark moment in the peace process that followed the violence of the Troubles.
Today, the only way you know you’ve come crossed from one country into the next is a change in road signs from miles – to kilometres-per-hour.
But what happens to the 310-mile stretch of land after the UK leaves the EU is already proving one of the biggest challenges in the Brexit negotiations.
FactCheck takes a look at the options, and what it could mean for the peace process.