Semaphore, from Greek σῆμα "sign, signal" and φόρος "bearer" describes a method of communicating simple messages over longer distances than voices can typically share the information. Semaphore is also useful when quiet is desired while communicating.

Modifications of semaphore include using lights, fires, mirrors to reflect sunlight, smoke signals, etc., have been useful throughout history. For Tolkien fans, the Phryctoriae were a semaphore system used in Ancient Greece for the transmission of specific prearranged messages. Towers were built on selected mountaintops, so that one tower, the phryctoria, would be visible to the next tower, usually twenty-miles distant. Flames were lit on one tower, then the next tower would light a flame in succession.

Semaphore has progressed into the digital age as a method of communication between digital devices. Semaphore is a type of synchronization in that realm.

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