Semaphore line

A semaphore line is an installation used to send optical signals. It is also known as optical telegraph. Common system include signal towers with special installations on them or hand-held flags. The meaning of the signal is determined by the position of the elements, for example the flags. The signal can be read when it is in a stable position. In modern usage, it refers to signalling with a set of flags.

Signal towers came before the electric telegraph. They were faster than sending out post riders with the information.

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  Optical telegraph of Claude Chappe on the Litermont near Nalbach, Germany.
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  Diagram of person holding 2 semaphore flags as letter "P"
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  A Chappe semaphore tower near Saverne, France
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  A Chappe semaphore telegraph at Louvre, France