That's interesting (we learn so much here):
In the Vietnam war Prisoner of War would authenticate a new prisoner's American identity by tapping the first 5 notes of "Shave and a Haircut," against a cell wall, waiting for the appropriate response. American POWs were then able to communicate securely with one another via the quadratic alphabet code (the Tap Code): http://en.wikipedia.org/wiki/Quadratic_alphabet_code
In the Vietnam war Prisoner of War would authenticate a new prisoner's American identity by tapping the first 5 notes of "Shave and a Haircut," against a cell wall, waiting for the appropriate response. American POWs were then able to communicate securely with one another via the quadratic alphabet code (the Tap Code): http://en.wikipedia.org/wiki/Quadratic_alphabet_code
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