The Tap Code is a code (similar to Morse Code), commonly used by prisoners in jail 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 code, not meant to avoid interception, since the messages are sent in cleartext.
1 2 3 4 5
1 A B C D E
2 F G H I J
3 L MN O P
4 Q R S T U
5 V WX Y Z
It was reportedly invented by four POWs imprisoned in Vietnam and is sometimes called "Smitty Code" after Captain Carlyle ("Smitty") Harris. Harris remembered an Air Force instructor who had shown him a code based on a five-by-five alphabet matrix (a Polybius square), as shown on the graph.
Each letter was communicated by tapping two numbers. The first designated the horizontal row and the second designated the vertical row. The letter "X" was used to break up sentences and the letter "C" replaced the letter "K".
For example, to communicate the word "WATER" the code would be the following
..... .. . . .... .... . ..... .... ..
Because of the difficulty and length of time required for specifying a single letter, most prisoners devised abbreviations and acronyms for common items or phrases.