Executive privilege. Words famously seared into history when Richard M. Nixon struggled to keep the Watergate tapes private in a criminal investigation.

In a political drama that held the nation in suspense, Nixon took his case to the Supreme Court in 1974, arguing that he had an absolute right of executive privilege to withhold information. The Supreme Court ruled unanimously against him and ordered him to turn over the tapes. Nixon resigned two weeks later.

Throughout the nation's history, presidents have repeatedly asserted executive privilege to keep secrets from the courts, the Congress and most anyone else.