The Engineer, 1966, C.C. Furnas & Joe McCarty, Eds.

More human-factors engineering has gone into the telephone than into any other non-military device. For example, when engineers were working on a new telephone handset in 1937 they measured 2,000 male and female heads before arriving at a diagram...showing the average-sized human head. Later the handset was given slightly greater curvature and shortened by about one half inch to bring the mouthpiece closer to the average caller's mouth.