A "Tap Code" allows you to communicate with only one kind of sound, e.g., metal on metal underwater, knuckles on wood, or clapping -- where Morse would be ambiguous. Prisoners in Vietnam used a version of a tap code to communicate between cells, with the letters of the alphabet in a 5x5 table. The "Optimal Tap Code" is similar to morse code, in that the shortest tap sequences correspond to the most common letters in the English language.
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