What secret communication method did American prisoners use in Vietnam?

The secret communication method used by American prisoners in Vietnam was known as Tap Code. This method allowed prisoners to communicate with each other through a series of taps or knocks, which represented letters based on a polybius square, effectively enabling them to send messages without drawing the attention of their captors. Tap Code was particularly useful in the confined spaces of prison cells, where verbal communication could lead to severe punishment.

This form of communication was discreet and reliable, allowing for the sharing of vital information, such as news about other prisoners or plans for escape, without risking detection. It served as a lifeline for morale and cohesiveness among the prisoners during a challenging time.