What to know about the president's 'designated survivor'

Spoiler: he won't be in the audience for Trump's address.

During major presidential addresses, the administration isolates one cabinet-level official in an undisclosed location. That person takes control if a disaster were to wipe out all those in the presidential line of succession.

Prior to the attacks on the Twin Towers on Sept. 11, the designated survivor had a relatively relaxed evening. One survivor recalled spending the night with his daughter, while another hosted a pizza party in the White House.

But post-9/11, security was beefed up: the designated survivor now undergoes hours of briefings and even practices disaster scenarios. Shortly before the president's speech, the designated survivor is whisked out of the nation's capital, accompanied by presidential-level security and a military aide carrying the "football," a briefcase that houses the nuclear launch codes.