Telegraph key - a general term for any switching device used primarily to send Morse Code characters. These keys are used for all kinds of manual telegraphy, for example, in the electric telegraph and radio-telegraphy.
The traditional telegraph key (Morse key) is a spring beam, swinging in the vertical plane, fastened to a steady base. By tapping on the end of the beam, the operator closes the circuit. The spring and beam ‘travel’ were regulated to provide the operator with ease of operation. Less common were the ‘horizontal beam’ devices. There are other devices with keys in which the contacts work simultaneously for circuit opening and closing.
The ability to work on mechanical key devices was considered a basic skill for the radio operator, regardless of how often it will be used in practice.