All biplanes, no matter by whom designed, have certain features in common. Besides the two superposed supporting surfaces from which they take their name, they all have a horizontal rudder or elevator, by means of which the machine is guided up or down and is prevented from pitching; a vertical rudder, by means of which the machine is kept on an even course and turned to the right or to the left; and some means by which the amount of main surface exposed to the pressure of the air can be varied, so as to keep the machine in balance from side to side. To these essential elements a tail, consisting of a small horizontal surface, is usually added, because it serves to steady the machine in flight.
Just how these elements shall be disposed is a matter of more or less difference of opinion among biplane designers, and this difference of opinion has given us the various biplanes of the Wrights, Curtiss, Farman, Goupy, Sommer, Bréguet, and others. Biplanes as a class follow the lines of the Wright machine. It is here impossible and unnecessary to describe in detail all the biplanes in use at the present day. For our purpose it will be quite sufficient to confine ourselves to the Wright, Curtiss, Farman, and Sommer machines, inasmuch as they represent the chief systems of control to be found in the two-surface machine.