Because of the difficulty and length of time required for specifying a single letter, prisoners often devise abbreviations and acronyms for common items or phrases, such as "GN" for Good night, or "GBU" for God bless you.
[3] The tap system simply requires one to know the alphabet and the short sequence "AFLQV" (the initial letter of each row), without memorising the entire grid.
[2][7] Harris had heard of the tap code being used by prisoners in World War II[8] and remembered a United States Air Force instructor who had discussed it as well.
[2][9] In Vietnam, the tap code became more widely used than Morse; despite messages taking longer to send, the system was easier to learn and could be applied in a wider variety of situations.
[12] They used it to communicate everything from what questions interrogators were asking (in order for everyone to stay consistent with a deceptive story), to who was hurt and needed others to donate meager food rations.
In the film The Ice Road, the tap code is used on a metal pipe conduit by trapped miners to communicate with executives of the mining company.
In the television show Breaking Bad, the character Hector Salamanca uses the version of a tap code shown at the right to communicate after suffering a stroke.