Their Navajo fluency was, without exception, strong — most had grown up in households where Diné Bizaad was the primary language, and the language was woven into every aspect of their daily lives.

The task of building the code was substantial.

The original twenty-nine men worked to develop a vocabulary of approximately 200 terms covering the military concepts they would most commonly need to transmit.

They road-tested the terms for clarity and memorability — a code word that could be confused with another in the stress of battlefield transmission was a potential liability, and the men who would be using the system under fire had to be able to rely on it absolutely.

The code had to be entirely memorized.

Writing it down was forbidden — a written copy of the code was a security risk that could not be tolerated.