The Good Friday Agreement, also known as the Belfast Agreement, was a political deal designed to bring an end to 30 years of violent conflict in Northern Ireland, known as the Troubles.
It was signed on 10 April 1998 and approved by public votes in Northern Ireland and the Republic of Ireland.
It set up a new government for Northern Ireland, representing both nationalists and unionists.
The agreement says that Northern Ireland is part of the UK and this can change only through a referendum.
The secretary of state must hold a border poll if it appears likely that a majority of people in Northern Ireland would vote for Irish unification.
There is no detail in the agreement on how the secretary of state should judge whether there is majority support – but they can decide to hold a border poll at any time, provided that there has not been another in the last seven years.
There is also debate as to what form a border poll should take, for example whether the poll should be held on the general principle of unification or after specific proposals have been set out for how a united Ireland would be governed.
