American POWs in Vietnam reportedly used a tap code based on a 5x5 matrix of letters. Each letter is encoded as the indices within that matrix, so "tap, tap" is

American POWs in Vietnam reportedly used a tap code based on a 5x5 matrix of letters. Each letter is encoded as the indices within that matrix, so "tap, tap" is A, "tap tap, tap" is B and so on, through to "tap tap tap tap tap, tap tap tap tap tap" for Z. The letters C and K share the index [3, 1]. https://en.wikipedia.org/wiki/Tap_code https://en.wikipedia.org/wiki/Tap_code