It consists of just five short, vaguely worded paragraphs, but Article 50 of the European Union's 2007 Lisbon Treaty will decide how Britain leaves -- and it is already causing problems.
The short section buried in the laws that govern the EU has never been used before and was written at a time when the prospect of any member state leaving seemed very unlikely.
The key opening phrase -- "Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements" -- was indeed the first time EU law laid out an exit plan.
But now it is at the centre of an acrimonious row between London and its soon-to-be former partners about how and when Britain will leave after Thursday's vote to quit.
"It provides few concrete details about how the withdrawal must be organised," Robert Chaouad, a research Fellow at the Institute for International and Strategic Affairs (IRIS), told AFP.
Article 50 says that "a Member State which decides to withdraw shall notify the European Council (the 28 leaders of the EU member states, led by EU President Donald Tusk) of its intention."
But it does not say when this must happen, and that has become the first crucial stumbling block.
Prime Minister David Cameron said on Friday he would resign by October, leaving his successor to begin the talks.
"I think it is right that this new prime minister takes the decision about when to trigger Article 50 and start the formal and legal process of leaving the EU," he said.
But his EU peers believe notification should be "as soon as possible" to minimise the chaos of a "Brexit", and preferably by Cameron himself at an EU summit on Tuesday and Wednesday.
What is clear from Article 50 is, however, that it is only the member state concerned which can make the notification. It cannot be forced on Britain by Brussels.
Jean-Claude Piris of the Delors Institute in Brussels said it was "normal and understandable" Cameron wanted to wait until a successor was in place, and that the rest of the EU "will not put a knife to Britain's throat".
"However if it drags on another six months or a year then it will become objectionable and I would understand if the EU became impatient," he told AFP.
Under Article 50, Britain's notification will set the clock ticking on a two-year period of negotiations within which a basic withdrawal agreement should be made.
After that "the treaties shall cease to apply to the state in question" -- or in layman's terms, Brexit is a reality.
The talks can in theory be extended if need be -- but only by the unanimous consent of Britain and the other 27 member states.
The alternative is a chaotic British exit on the stroke of two years, with lots of loose ends untied.
Britain and the EU will separately need to negotiate what Article 50 calls their "future relationship".
This is not spelled out but would include issues such as access to the single market, whether Britain will have trade deals with the EU, whether free movement will continue, and so on.
Under Article 50, negotiations on these topics will take place in parallel with the talks on the basic withdrawal agreement.
But again it is vague, mentioning only the talks "taking account of the framework for its future relationship with the union".
"Legally, everything is possible, because there is no precedent. One can imagine a withdrawal agreement that would double as an agreement establishing relations between the UK and the EU," said Chaouad of IRIS.
Article 50 does say that a former EU nation can seek to rejoin after leaving -- under Article 49 of the Lisbon Treaty.
This however begins the membership process from zero, in the same way as current candidate states like Turkey, Serbia and Albania.
The remaining 27 EU states must approve Britain's withdrawal agreement by a "qualified majority". The European Parliament will also vote by a simple majority.