Early during World War II, British intelligence officers eavesdropped on German radio transmissions, but because the messages were in an encrypted version of Morse code, the British couldn’t understand the content. The dots and dashes came in distinctive rhythms, and the Allied spies quickly learned to recognize each Morse code operator’s particular style, which the listeners called the operator’s “fist.”
Having identified the individual code senders, the intelligence officers triangulated signals and traced the operators’ movements across the continent—thus tracking the movement of their military units.