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.
The following semaphore characters are presented as one would face the flagman: