My under standing is a D cross section puts more compression force on the belly than a retangular one.I agree that any decent bow wood will make a D cross sectional bow if made long enough for the wood species in question.not to give bad advice that only osage,yew or elm will make a D,but I think one would have a better chance of sucess using known compression strong wood and be less likely to chrsal or frett due to and uneven tiller of a novice.