Communication by means of a telecommunication system that transmits documentary matter, such as written or printed matter or fixed images, and reproduces it at a distance. The matter is transmitted as a suitable signal code by means of wire or by radio (radio telegraphy). A telegraph network is a complete system of stations, installations, and communication channels that provides a telegraph service. Telex is an example of a telegraph service.

Telegraphy may be effected automatically or manually. A synchronous system is one in which the sending and receiving instruments operate at substantially the same frequency and are maintained with a desired phase relationship. Transmission may be in the form of a direct current applied to the line by the sending apparatus or a modulated carrier wave. Amplitude, frequency, or pulse-code modulation of the carrier can be used. Subcarrier modulation is a method that may be employed in radio telegraphy when a low-frequency carrier wave (the subcarrier) is frequency-modulated by the telegraph signal and then this modulated wave is used to modulate a second radiofrequency carrier wave.