Code 24 : Tap Code

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	M	N	O	P
4	Q	R	S	T	U
5	V	W	X	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.

The Tap Code

One of the most important parts of a POW's life was communicating with his fellow captives. The first communication between isolated prisoners of war may have been a name scrawled on a piece of toilet paper with the burnt end of a matchstick. Notes and whispers were attempted, but both were often detected and severely punished.
In June 1965, four POWs -- Captain Carlyle ("Smitty") Harris, Lieutenant Phillip Butler, Lieutenant Robert Peel and Lieutenant Commander Robert Shumaker -- who were imprisoned in the same cell in Hoa Lo devised a simple, secretive code. The four men, expecting to be split up again, vowed to continue their resistance. To do so, they knew communicating closely would be essential.
Harris remembered an Air Force instructor who had shown him a secret code based on a five-by-five alphabet matrix. Each letter was communicated by tapping two numbers: the first designated the horizontal row and the second designated the vertical row. The letter W, for example, would be 5-2; the letter H would be 2-3. The letter x was used to break up sentences and the letter "c" replaced the letter "k." (One of the famous, yet inelegant, usages of the letter "c" for "k" was in the transmission "Joan Baez Succs," which POWs sent around the camp after the American anti-war activist's songs were played over the camp's public address system.) Here is the way the alphabet code was set up:
The guards separated the four prisoners after one was caught passing a note, and thus inadvertently spread the code, as the separated men taught it to others. By August 1965, most of the prisoners had been initiated, and were passing messages by tapping on the walls to fellow prisoners. "The building sounded like a den of runaway woodpeckers," recalled POW Ron Bliss.