The Union這個從美國南北戰爭留下的密碼盤，直徑大約95mm，它由兩個同心可旋轉的盤子裝在一的中心點所組成。圓周被等分成三十等分 The smaller inner disk contained letters, terminations and word pauses, while the outer disk contained groups of signal numbers. For easier recognition, the number eight represented two. The initials A.J.M. represent the Chief Signal Officer General Albert J. Myer. Each disk had a control number used for accountability.
