Article 50 is the European Union legislation that sets out how a member state can leave the organization. It's part of the Treaty of Lisbon, which was signed in 2007 and went into force in 2009.
What does it actually say?
1. Any Member State may decide to withdraw from the Union in accordance with its own constitutional requirements.
2. 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.
3. 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.
4. 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.
5. If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49.
What does that actually mean?
The law is vague on the specifics, perhaps intentionally — no country has ever invoked Article 50 in the past, and the E.U. long viewed the possibility of a country leaving as an unlikely and unwanted possibility.
The basics are covered, however. There is no set way for a country to decide it wants to leave the E.U. The member state seeking to leave can decide that itself. Then it will have to give an official statement to the European Council explaining that it plans to leave. This is what May will do on March 29.
The country that wants to leave would not negotiate directly with other member states to reach a deal on how it would leave. Instead, the 27 other member states would meet as the European Council and agree on a framework. Britain would then negotiate the technical details with the European Commission. The European Parliament also will have a say, giving consent to the deal.
The article also clearly states that a country will have two years to reach an agreement on the exit, during which time the country would still be governed by E.U. treaties and laws, although it would not be allowed in the decision-making process. However, if all E.U. member states agree, that deadline could be extended.
Finally, if the country later decides it wants to rejoin the E.U., it has to apply for membership just like any other nation would.
Why trigger Article 50 now?
Originally, then-British Prime Minister David Cameron had suggested he would trigger Article 50 immediately after Britain's June 23, 2016, vote to leave the E.U. However, that move was swiftly delayed by a number of factors, including Cameron's own resignation and the subsequent leadership battle to replace him.
May, Cameron's eventual successor, said in October that she wanted to trigger Article 50 by the end of March. There were further complications, however. A legal challenge meant that she was unable to use “royal prerogative” to force Britain to trigger Article 50 and instead was required to get Parliament's approval. While the bill went through the lower House of Commons, the upper House of Lords tried to force amendments before backing down.
Logically, it makes sense for May to try to get the ball rolling on Article 50. Britain's economy is already at risk because of uncertainty over the country's future, so prolonging that uncertainty is a problem. But there is also a logic in not immediately triggering Article 50 — the negotiations are going to be tough for Britain, so it makes sense to take some time to work out the British position before formal talks begin.
What will these negotiations involve?
The European Council will draw up guidelines for the negotiations. Both Britain and the E.U. will have large teams to negotiate. Britain has the newly formed Department for Exiting the European Union, which is led by Member of Parliament David Davis, with Trade Secretary Liam Fox and Foreign Secretary Boris Johnson also likely to take large roles. The European Commission has created a task force headed by French politician Michel Barnier.
The negotiations themselves will be wide-ranging, with unspent E.U. funding, the future of E.U. nationals living in Britain and security arrangements all likely to be involved. It is currently unclear whether the negotiations will include a future trade deal between Britain and the E.U. or whether that will be handled separately.
Any end deal will have to be approved by a “super” qualified majority (more than 72 percent) in the European Council, and it would also need the approval of the European Parliament. May has also suggested that Britain's Parliament will have a say on the final deal.
What else will Britain have to do?
A lot. It will have to work out not only its new relationship with the E.U. but also new trade relationships with many countries around the world. Experts suggest these separate negotiations could take years.
The British government will also have to repeal the legislation that took it into the E.U. and convert E.U. law into British law. There are reports that a “Great Repeal Bill” that would include both of these elements may be revealed on the same day as Article 50 is triggered. However, it is the subject of some controversy, as it may rely on little-used powers known as the “Henry VIII clauses.”
There may be other events for May to watch out for — including but not limited to another potential Scottish independence referendum and perhaps an early British election.
So will Britain be out of the E.U. by March 2019?
That's the plan. Whether it happens that way is harder to say. Many experts suspect that since the Article 50 process has never been implemented before, it may take a long time to go through all the details. If no deal is reached within two years, it is possible that Britain would be forced into what has been dubbed a “dirty Brexit.” Even if it does take less than two years, it may result in only a transition deal, with the hard work of reimagining Britain's relationship with the E.U. still to come.
It is possible to extend the negotiating period further, but only if all 27 member states agree. And, yes, most experts seem to think that it will be possible to reverse Brexit if Britain can convince the E.U. it has changed its mind.