Tap Code
The Tap Code is a manual cipher system used for encoding messages through a series of taps, knocks, or other simple signals, and was popularized for covert communication among prisoners of war in the 20th century, particularly during World War II and the Vietnam War. It is essentially a numeric substitution cipher based on a 5×5 Polybius square in which letters are assigned coordinates, with C and K sharing a single position.
Semaphore Cipher
The Semaphore cipher is a visual signaling system that encodes letters as distinct physical positions rather than transforming text through mathematical substitution. Although often grouped with ciphers for convenience, Semaphore is more accurately described as a manual encoding system, where meaning is conveyed by the position of flags, arms, or mechanical indicators.