The Ultimate Designated survivor - American History Information Guide and Reference

Because of the remote possibility of a catastrophic event which could wipe out large portions of the United States' federal government, the entire cabinet is rarely gathered in one place at the same time, in order to maintain continuity of government with regard to presidential succession.

During State of the Union Addresses and presidential inaugurations (which almost all top-ranked federal officials attend) one member of the cabinet, the designated survivor, is kept in a separate, secure, and undisclosed location apart from the rest of the Cabinet in the event of a catastrophic event. It is a practice that originates from the darkest days of the Cold War and fears of a nuclear attack. Since the terrorist attacks of September 11, 2001, one Representative and one Senator of each political party have served as designated survivors concurrently with the Cabinet member, so they can ascend to the office of Speaker of the House of Representatives or President Pro Tempore of the Senate (the offices that are respectively first and second in the line of succession to the presidency after the Vice President).

List of designated survivors