Control-Alt-Delete was never meant for the public. In 1981 in Boca Raton, Florida, IBM engineer David Bradley was writing the ROM BIOS for the IBM PC 5150, and the prototype crashed constantly. Every crash meant a hard reboot and a slow power-on self-test of every kilobyte of memory. Bradley spent five or ten minutes coding a keyboard interrupt that would skip the test and restart the machine, then moved on to the next crisis.

This episode follows that hidden shortcut out of the lab and into daily life. IBM printed it in its technical reference manual, users found it, and Microsoft had to manage it: first as an early task manager in the multitasking era, then as the secure attention sequence in Windows NT, where it defeated fake login screens by reaching the kernel directly. Along the way come Bradley’s famous jab at Bill Gates and Gates’s later admission that the three-key login was a mistake.

• Bradley’s first choice was Control-Alt-Escape, but those keys sat together on the left side of the keyboard and developers kept triggering resets by accident.
• Chief programmer Mel Hallerman suggested swapping in the Delete key, which sat on the far right and made the command impossible to press with one hand.
• The text on the Windows 3.1 blue screen that appeared after the keystroke was written by Steve Ballmer.
• At the 20th anniversary of the IBM PC in 2001, Bradley said: I may have invented it, but Bill made it famous.
• In 2013 at Harvard, Gates said Microsoft wanted a single dedicated login button but could not convince IBM’s hardware designers to add the key.