Consider the heavy vaults of 19th-century London. To open them, a single key wouldn't cut it—bankers engineered a "two-man rule" requiring two distinct keys held by two separate officers to turn the lock simultaneously. Builders knew that if a single lock or outer wall failed, a secondary barrier was the only thing standing between safety and total collapse. Even so, fast-forward to today and many businesses are still relying on the electronic equivalent of a simple wooden latch: a lone password.