By Daniel J. McLaughlin
When Theresa May triggered Article in March 29 last year, the prime minister officially notified the European Union of Britain's departure - and started the clock for a two-year negotiating period. Once it started, it couldn't stop. With the deadline looming ominously, time is a luxury for the negotiating partners. And as a no-deal Brexit becomes more of a bleak reality, there are those who wish that Article 50 would have granted more time.
Article 50 comes from the Lisbon Treaty, which became law in December 2009. It followed a process by European leaders to make the EU "more democratic, more transparent and more efficient". The article in question was a very basic five-point plan should any country wish to leave the EU.
What is Article 50?
It states: “Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.
"A Member State which decides to withdraw shall notify the European Council of its intention. In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State, setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union.
"That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament.
"The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after the notification referred to in paragraph 2, unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.
"For the purposes of paragraphs 2 and 3, the member of the European Council or of the Council representing the withdrawing Member State shall not participate in the discussions of the European Council or Council or in decisions concerning it. A qualified majority shall be defined in accordance with Article 238(3)(b) of the Treaty on the Functioning of the European Union.
"If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49.”
Can it be reversed?
The two-year negotiating period is not enough time to sort out a future relationship with the bloc. Leading EU law professor Michael Dougan told BBC Newsbeat: "The overwhelming consensus is that these things do not take two years to negotiate, the rough guide that we are all talking about in the field is around 10 years.
"The treaty said that you have two years within which to make your divorce settlement. But the divorce settlement is completely separate from the framework agreement for your future relations with the EU."
The terms of Britain's departure will be negotiated between the EU27 and the UK, and each of the EU member states will have a veto over the conditions. The EU27 will also ratify the Brexit deal in their national parliaments, as well as Britain turning to its own parliament for an agreement. This could potentially mean, for example, French MPs could choose to hinder the entire process, if they want to.
Lord Kerr, who was Secretary General of the European Convention when the Lisbon Treaty was agreed, argues that the government can reverse Brexit if it wants to. He notes that Mrs May's letter, informing the EU of the UK's intention to withdraw, was exactly that - an intention. Intentions can change. "The Article  is about voluntary withdrawal, not about expulsion," he argues.
The former diplomat writes: "The clause that says that "once we're out, we're out" says just that, and only that. If we had wanted an intention to go to be the Rubicon moment, if we had wanted a notification letter to be irrevocable, we would have drafted the clause to say so. But we didn't, and the clause doesn't. So, the die is not cast irretrievably. The letter can be taken back."
The Daily Telegraph asks whether Article 50 is reversible and comes up with different answers. Article 50, it concludes, is a piece of European law, and therefore the EU's European Court of Justice would have a final say on whether it could be reversed. "And therein lies the unsatisfactory, but most accurate, answer to the question: the UK might be able to revoke Article 50, but only if it gets permission from the EU."
If Article 50 is not reversed, Britain will leave the European Union on March 29, 2019 at 11pm (GMT).