UK voters have opted to leave the European Union, but the UK hasn't left just yet. In fact, it's quite a drawn-out process.

The only legal way for a Brexit – or for any member state to withdraw from the European Union – is by triggering an obscure and controversial clause in the Lisbon Treaty: Article 50. It gives the departing country two years to negotiate the terms of its withdrawal and has never been used before. Tom Clark of The Guardian explains how it works.