The Blueprint of Hope: The 5×5 Grid

The genius of the tap code lies in its profound simplicity. It’s based on a 5×5 grid known as a Polybius square, a cipher system that dates back to ancient Greece. The grid arranges the Latin alphabet into a matrix, allowing any letter to be identified by its row and column coordinates.

The prisoners used a standard version of this grid, with one small but crucial modification. To fit the 26 letters of the English alphabet into 25 squares, they combined ‘C’ and ‘K’. If a word needed the ‘K’ sound, they simply spelled it with a ‘C’. For example, the word “KICK” would be tapped out as “C-I-C-C.” The captors would never know the difference.

The grid looked like this:

    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

This simple chart became the foundational text for their secret language, a key that could unlock the silence.

From Grid to Grammar: How to “Speak” Tap

Communicating with the code was elegantly straightforward. A prisoner would tap out two numbers for each letter: the first number corresponded to the row, and the second to the column.

• A short pause separated the row taps from the column taps.
• A longer pause separated each letter.

Let’s say a prisoner wanted to tap the letter ‘G’. Looking at the grid, ‘G’ is in the 2nd row and the 2nd column. The communication would be:

TAP TAP (pause) TAP TAP

To spell out a simple word like “HI”:

• H: Row 2, Column 3 → TAP TAP (pause) TAP TAP TAP
• (longer pause)
• I: Row 2, Column 4 → TAP TAP (pause) TAP TAP TAP TAP

The medium was whatever was available—a knuckle against a wall, a broom handle swished in a bucket, a cough, a scrape of a foot. The rhythm was everything. New prisoners were often taught by a veteran in an adjacent cell, who would patiently tap out the entire grid, letter by letter, until the “newbie” had it memorized. The initiation often began with a simple call-and-response: the rhythmic five taps of “shave and a haircut.” A new American prisoner would instinctively know to reply with the concluding “two bits”, establishing a connection and opening the door to learning the code.

Whispers Through the Walls: Stories of Resilience

The tap code was far more than a simple cipher; it was the social and military fabric of the POW community. Its inventor in the camps is widely credited as Air Force Captain Carlyle “Smitty” Harris. Shot down in 1965, Harris remembered the code from a survival training lecture and, tapping through a wall to a fellow POW, began to spread it throughout the prison system. It became mandatory for every prisoner to learn and use.

Through this system, the senior ranking officers (SROs) could reestablish the military chain of command. They issued orders, coordinated resistance efforts, and gathered intelligence. When a new prisoner arrived, he would be debriefed via the tap code, providing crucial information about world events, the state of the war, and names of other captured servicemen.

More importantly, the code was a tool for survival. Prisoners shared information on their captors’ interrogation techniques, identifying guards who were particularly brutal or those who could be subtly manipulated. They warned each other about upcoming inspections and coordinated stories to tell their interrogators.

But its most vital function was preserving sanity. The code broke the crushing weight of solitary confinement. Men shared life stories, told jokes, and prayed together, all through a series of rhythmic knocks. They would tap “GN” (Good Night) and “GBU” (God Bless You) to each other before sleeping. These simple messages were a powerful affirmation that they were not alone, that they were still part of a community, and that someone cared if they lived or died.

Commander Everett Alvarez Jr., one of the longest-held POWs, later recalled, “It was a way of connecting with each other, of being human. We were social beings, and we needed that contact. The tapping was our way of saying, ‘I’m still here, and I’m with you.'”

A Language of Unity and Defiance

From a linguistic perspective, the tap code is a fascinating case study. While technically a cipher for English, it evolved its own unique characteristics. The development of abbreviations like “GBU” is a classic example of linguistic efficiency, similar to modern text-speak. The rhythm and pauses functioned as a form of syntax, separating characters and words with temporal punctuation.

This language was a profound act of defiance. The North Vietnamese aimed to isolate and demoralize, to turn each prisoner into a broken, solitary unit. The tap code completely subverted this goal. It transformed separate cells into a network, isolated men into a cohesive unit, and the silence of the prison into a “loud” and constant stream of information, humor, and hope. It was a language the guards could hear but never understand, a secret world flourishing right under their noses.

The story of the POW tap code is a powerful reminder that language is more than just words. It is connection, community, and resistance. It is the tool we use to build worlds, even within the confines of a prison wall. The simple, rhythmic knock of a knuckle against concrete became the heartbeat of the POWs in Vietnam—a sound of hope, of courage, and of the unyielding human spirit finding its voice in the dark.