After months of negotiations, the European Union and the United Kingdom have reached a deal on a transition period that will follow Britain's departure from the continental bloc. The union's chief negotiator, Michel Barnier, and his British counterpart, David Davis, announced the agreement during a joint press conference on March 19.

The transition period will start March 2019, once the United Kingdom has formally made its Brexit, and will end December 2020. During that time, the United Kingdom will remain a member of the EU single market, the area where goods, services, people and capital move freely. But crucially for London, the British government will be allowed to negotiate free trade agreements with non-EU countries once the transition period starts. EU citizens will still be allowed to move to the United Kingdom and their rights will be protected by the European Court of Justice, which is a key request by Brussels. Moreover, London has conceded to allow EU fishermen to maintain their access to British waters during the transition.

This latest agreement marks significant progress in the Brexit process, but leaves an important question unanswered: the future of Northern Ireland. During the press conference, the United Kingdom and the European Union restated their commitment to keeping the Irish border open after Brexit but offered no new details about how they plan to do that. The two sides confirmed their previous agreement that Northern Ireland would stay in the EU customs union unless a different solution is found. But the British government is strongly opposed to allowing Northern Ireland to remain the customs union, which means that London will insist on finding ways to avoid a hard border. It will likely push for the two sides to consider options such as regulatory alignment with the European Union and the use of technology, opening the door for new frictions. Davis admitted that there was not yet an agreement "on the right operational approach." Meanwhile, Barnier warned that Brexit negotiations take place under the principle that "nothing is agreed until everything is agreed," meaning discord over the Irish border could compromise progress.

Now that the two sides have settled on the transitional agreement, the European Union will debate over and likely approve the document during its upcoming summit March 22-23. After that, the United Kingdom and the union will begin conversations about their future trade relationship. Though the European Union can't legally sign a trade agreement with a member state, there is nothing preventing the parties from discussing a future deal. Davis said that London will aim to sign "the most comprehensive trade deal ever" with the bloc, but this will be easier said than done. For example, the European Union has so far refused to include financial services in the agreement as London desires. EU officials have suggested that the future EU-U.K. free trade agreement will probably resemble the one that the bloc has with Canada, where most goods but only a limited number of services are included.