The terminology used to describe the major features of a typical double-bay biplane of the World War I time period is illustrated in figure D.1. The number and arrangement of struts and wires employed in biplane design have varied greatly over the years; however, the terms indicated in the figure have survived and are still in use today in any discussion of modern-day sport or agricultural biplanes. A single-bay biplane, in contrast to the two-bay arrangement shown in figure D.1, has only one set of interplane struts between the wings on either side of the fuselage, and a triple-bay design has three sets of such struts on either side of the fuselage. In contrast to the incidence wires shown in the figure, many biplane designs have utilized a single strut, in an "N" arrangement, connecting the front and rear interplane struts (see the Travelair 4000 in figure 4.5, for example). This configuration eases the task of rigging, or aligning, the wings in the correct relation to each other and the fuselage. The proper rigging of wire-braced aircraft once formed an extremely important part of any aircraft erection, maintenance, or repair operation. Today, the experienced rigger is almost extinct, and the art is all but lost except for a few dedicated enthusiasts engaged in the restoration of antique aircraft or in the building and flying of sport biplanes. Decalage is a term, not illustrated in the figure, that is sometimes encountered in discussions of biplanes. It refers to the difference in angle of incidence at which the upper and lower wings are mounted on the aircraft.
More may be found on biplane terminology, construction and assembly in references 26 and 79.