It's the European Union's exit door, the legal way out of the bloc.

Known as Article 50, this exit mechanism was first enshrined in the EU's governing Treaty of Lisbon in 2009 and has never been used before. Here's a look at how it works following Britain's vote to leave the EU:

WHAT IS IT?

Article 50 is short: It includes just five main points on less than half a page, but it is long on ramifications.

Essentially, it says that any member can leave the EU according to its own constitutional requirements.

That member should notify its EU partners in an unambiguous way of its intention to trigger this "exit clause," setting off a two-year window in which the formal departure will be negotiated.

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WHEN WILL BRITAIN ACTIVATE IT?

This is the question on everyone's lips in Europe in the wake of last week's politically earth-shattering referendum in Britain.

British Prime Minister David Cameron says he's resigning and it's up to his successor to notify the EU of the country's departure, which could be in October — or later.

Some have speculated that the next prime minister may not trigger it at all. But as far as European Council President Donald Tusk is concerned, it's the only option on the table, and Britain can't be forced out until it's good and ready to go.

"It is the British government who initiates the process of exit from the EU. And this is the only legal way we have," Tusk told reporters Tuesday before chairing a two-day EU summit in Brussels.

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WHAT HAPPENS NEXT?

Once London triggers the exit clause, the remaining 27 members of the EU will draw up the guidelines for it to happen and must accept Britain's departure by a qualified majority vote - around two-thirds. Should the talks bog down, the U.K. could ask for an extension beyond the stipulated two years, but all 27 others would have to agree.

The European Parliament must give the exit its consent too. Until it all happens, Britain will remain a member, with all the rights and obligations of membership.

The exit negotiations will take into account how the two sides see their future relationship. But they cannot get into the nitty gritty of those ties, like detailed trade talks, until the U.K. is actually out, possibly in early 2019.

If it changes its mind down the road, it can ask to join again. The EU's entry door is Article 49, but that's another story.