At the time of the French Revolution, the French technician Claude Chappe succeeded in developing a technically practical optical telegraphy device based on the transmission of characters by means of pivoting signal arms (also known as a wing telegraph or semaphore).
Two pivoting crossbars were attached to a high mast, with two further pivoting bars at each end, by means of which different letters could be signaled on the basis of a code, depending on their position.
Several series of tests showed that the system was easy to operate and robust.
Thus, in 1794, the first regular telegraph line was established between Paris and Lille, covering 270 km with 22 semaphore stations.
The running time for the transmission of a single letter was an impressive two minutes at the time. The flexibility and speed convinced the military in particular to quickly set up a nationwide optical-mechanical telegraph network.
For message transmission, the crossbar had to be horizontal, vertical or diagonal.
The signal arms could each project at an angle of 45°, 90°, 135°, 225°, 270° and 315° or be folded back onto the crossbar.
This resulted in a total of 7 – 7 – 4 = 196 signals.
Of these, 104 were used for transmission control and 92 for message transmission.
A codeword consisted of two consecutive signals, so that 92 – 92 = 8464 codewords were available.