Despite the new methods of transmitting messages produced by the war, the telephone remains the backbone of the military signal system. Though the portable telephone instrument used by all front-line troops was manufactured in the United States for commercial purposes prior to the war, the switchboard in most general use by mobile troops was originally developed by the French, being the only telephone equipment used by the American forces which was not of American design. This switchboard, which was built in units so that it could be expanded from four to twelve lines, was the “Central” of the front-line dugout, being so compact that it could be carried as part of the equipment of a soldier and quickly put into operation. For the use of the larger field units there was designed a camp switchboard, with provision for forty wires, which when in transit resembled a commercial traveller’s sample-trunk. A third type of switchboard, for use at headquarters in the zone of combat, but where extreme portability was not essential, was designed in units, like a certain popular style of sectional bookcase, and could readily be increased to any size required. An important auxiliary to the field-telephone lines was the buzzerphone, an American device for use where extraordinary secrecy was imperative, it being impossible for the German Listening-in Service to eavesdrop on messages sent by this method. [Illustration: COMMUNICATION BY USE OF PANELS. When other means of communication is found impracticable, the infantry can communicate with aviators by means of panels of cloth cut in various shapes spread upon the ground. _Photograph by Signal Corps, U. S. A._] [Illustration: A MEMBER OF THE SIGNAL CORPS SENDING MESSAGES BY MEANS OF A LAMP.] Prior to the war the “lance-pole” was used exclusively by American troops in the field, as it permitted of rapid line construction and served its purpose admirably in open warfare. The conditions prevailing in Europe made the use of this pole impracticable, however, and where poles were used at all they consisted of very short stakes with special cross-arms, miniature copies, in fact, of the commercial equipment commonly used in the United States. The enormous mileage of the trench-lines called for vast quantities of insulators, cross-arms, and other special fittings, in all of which there was great wastage, for though the instruments used on the military lines usually had a certain degree of protection, the lines themselves were constantly exposed to artillery and airplane bombardment.