The White House announced on Tuesday that it was seizing control of the press pool covering President Trump, becoming the first administration in decades to insist that it will decide which news organizations, and which reporters, are allowed to be close enough to question the president in places like the Oval Office and Air Force One.

The decision raises questions: What is the press pool? Why does it exist? Why is this different from what has happened in the past?

Here is what to know:

What is the press pool?

The White House press pool was created during President Dwight D. Eisenhower’s term to solve a practical problem: When the president makes news on the road or at the White House, which of the hundreds of White House correspondents get to be there?

The answer, developed through cooperation between news organizations and presidential administrations of both parties, was the creation of a small group of correspondents, TV cameras and photographers who would be the eyes and ears of the broader White House press corps in those small settings.

The arrangement works like this: The White House agreed to let a group of reporters be with the president in those locations, while the choice of which correspondents were in the group was left to the White House Correspondents’ Association. The W.H.C.A., created in 1914, is an independent group whose members are accredited to cover the president.