The keys were picked to be hard to hit by accident. Wikipedia says Bradley first tried Ctrl, Alt and Esc, but found it was too easy to bump the left side of the keyboard and reboot the computer accidentally. Wikipedia says the switch to Delete made it a combination impossible to press with just one hand on the original IBM PC keyboard.

Years later Gates said it was a mistake. ABC News reported that at a Harvard event in 2013 he said "it was a mistake."