The term calcar femorale (originally referring to a bony spur projecting into the cancellous tissue of the base of the femoral neck) differs from most anatomical terms in that it has developed separate meanings in different areas of use. In particular, its present meaning for most orthopedic surgeons differs from its classical anatomical significance. This work shows that the classical, "textbook" anatomical interpretation-usually attributed to Merkel (1874, cited by Harty, 1957, J. Bone Joint Surg. [Am.] 39:625-630)-is not entirely correct, being based on an incomplete appreciation of the true nature of the three-dimensional structure of the upper end of the femur. Though Humphry (1858, A Treatise on the Human Skeleton, Cambridge, England: Macmillan) was aware of the importance of the third dimension, it was the largely neglected work of Dixon (1910, J. Anat. Physiol., 44:223-230), itself published to draw attention to the ideas of Krause (1909, in Bardeleben's Handbuch der Anatomie des Menschen, Jena: Gustav Fischer), which indicated the true nature of the calcar femorale and the limitations of Merkel's view. Dixon's work has been developed and consolidated by others such as Garden (1961, J. Bone Joint Surg. [Br.] 43:576-589), who was apparently unaware of Dixon, just as Dixon himself appears to have been unaware of the work of Bigelow (1875, published in Bigelow, 1900, The Mechanisms of Dislocations and Fracture of the Hip, Boston: Little, Brown), which pre-empted one of his major concepts. Even earlier work by Bigelow (1869, published in Bigelow; 1900, The Mechanisms of Dislocations and Fracture of the Hip, Boston: Little, Brown) appears to have pre-empted that of Merkel. The adoption of the three-dimensional, "Dixon concept" of the bony anatomy of the upper end of the femur leads to an entirely different set of structural engineering analogies from those two-dimensional ones which have become commonplace since the work of Ward (1838, cited by Garden, 1961, J. Bone Joint Surg. [Br.] 43:576-589) and Wolff (1870, cited by Keith, 1919, Menders of the Maimed, London: Frowde and Hodder & Stoughton).