The Brexit vote becomes real
What was an obscure, technical and legal term today becomes a political move that will change the country.
A letter signed by the prime minister on the green baize table of the cabinet room yesterday afternoon makes real the consequences of Britain's vote to leave the European Union nine months ago.
Once the document arrives in Brussels at lunchtime, passed formally into the hands of the European Council, the triggering of Article 50 begins the process of Britain leaving the European Union - a partnership of nations in which the UK has played its own role for more than four decades - for good, or ill.
The prime minister will promise later to represent every person in the country during what are likely to be fraught negotiations, including those EU nationals who have made their homes here, whose future is still uncertain.
Theresa May will also urge the country to come together, hoping this moment could spell the end of a fractious debate between Leave and Remain. The government's main priorities are clear - withdrawing from European law, controlling immigration and striking a free trade deal from outside the European single market.
Yet there are tensions in Parliament, in the prime minister's own party, between Holyrood and Westminster, and of course, among the public over what Britain's future could, or should look like outside the European Union. She, and we, have two years to work it out.
Theresa May of course was a Remainer to start with, if not the most full-throated advocate for the EU during the referendum campaign.
But after the bloody Tory infighting in the campaign's immediate aftermath, she is sometimes described as being the "last grown-up left standing".
There are doubts in Westminster about the government's capacity to deal with the complexities of what lies ahead, doubts about the Tory Party's capability of sticking together when it gets tough, doubts about the opposition's ability to carry out the kind of intense scrutiny required while this vital set of decisions are debated and discussed over the next two years or so.
Above all perhaps, doubts about whether what Theresa May is asking for is even vaguely realistic. What she does or does not achieve in these negotiations will determine her, and the country's, future.