A knowledge of the International Radio Code is essential in air operations, as it is the standard means whereby the airplanes may communicate with the ground. It is therefore necessary to train not only pilots and observers, but also all radio mechanics and operators thoroughly in the use of this code, in order that efficient results may be obtained in field service. In this pamphlet will be found described in detail description of apparatus and methods used in the instruction of personnel of the Air Service in code. While intended primarily for the use of instructors at Communications Schools, this pamphlet will be found of considerable assistance to commanders of newly organized units in continuing the training of the personnel thereof. It will be found of assistance also in carrying on vocational training within a command.