This is the kind of thing that gets hard to define because of published exceptions. Traditionally, 'D' bows would bend through the handle and in most cases the handle wouldn't be narrowed (see, 'perfect bow under 50# in the bowyer's bibles). For some reason Dean Torges describes such bows as 'C' bows and bows with stiff, narrowed handle sections and elliptical tiller 'D' bows in his book 'Hunting the Osage Bow.' I'm not completely sure if there is any historical precedent for Dean's definitions or if he was just trying to create new terms that he thought were more useful. At any rate, sometimes people use the terms 'C' and 'D' to refer to cross-sections, and at other times referring to tiller profile. There is no universally accepted definition for 'D' bows but the definitions given by those above are accurate and represent 95% of use. Just beware of the rare exception.
J. D.