In addition to the foregoing, there are some special types the distinguishing features of which merit reference. Many other types of successful biplanes and monoplanes are in use, but they differ so slightly from one or another of those described here that any detailed mention of them would only lead to confusion. The great number of machines now being built in this country by individual experimenters or by manufacturers are either replicas of those detailed or are modifications of them. Nomenclature. Despite the phenomenally rapid development of aviation, its terminology has kept pace so that there are a number of expressions the meaning of which must be explained before attempting a description of the machines themselves. Supporting plane. By supporting plane is meant the main lifting surface as distinguished from all auxiliary or stabilizing surfaces. Direction and elevation rudders. Direction rudder refers to the movable, vertical surface used for steering to the right or left, while the eleva tion rudder is a horizontal surface the function of which will be obvious. Transverse control. Transverse control is the device employed for the preservation of lateral balance when ?ying straightaway and for maintain ing an artificial inclination of the machine when rounding turns. Keels. Cls are fixed surfaces intended to aid in the preservation of stability; they exert neither lifting effect nor rudder action. Spread. Spread is the maximum horizontal dimension perpendicular to the line of ?ight.