It may not be out of place here to consider what Constitutes a plane, as that term is used in aviation. It is that part of the aeroplane, the pressure of the air upon the surface of which, lifts and sustains the aeroplane aloft. The plane may take a variety of forms; it may be curved or its parts may meet in an angle; it may be uniform and unbroken in shape or divided into parts. The two wings of a bird would constitute a monoplane, when they are in a horizontal position for soaring, or when the tips are uplifted and they form an angle like a broad V, called a dihedral angle. If the aeroplane has two such planes, one back of the other, it is still called a monoplane, or, more definitely, a tandem monoplane; but if one of the planes is above the other it is called a biplane. A similar arrangement of three planes, one above the other, could be called a triplane and one of several planes a multiplane.