The tap code is a cipher, commonly used by prisoners to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple cipher, not meant to avoid interception, since the messages are sent in cleartext. United States prisoners of war in North Vietnam during the Vietnam War are most known for having used the tap code, although its use dated back to as early as the Korean War