The Tap Code: Talking Through Prison Walls In prison camps, men fell back on the oldest grid in signalling: the tap code, five by five, knocked through walls. This guide explains the Polybius square's two-thousand-year career as the code that cannot be confiscated. Quick facts Grid five by five Sent as row taps, column taps C and K share one cell Carried names, news, rolls Needs no equipment The tap code's virtue is that it cannot be taken away. Used in Korea and later in Vietnam, it preserved the one thing captivity attacks first - knowledge of each other. It is two thousand years old and has never been improved, because it cannot be. Go ahead: https://morsecodeworks.com/ Morse Code Works - free Morse code tools and guides. Reference chart