From “The Practice and Science of Drawing” by Harold Speed: “Nothing is more awful than shadows darker in the middle and gradually lighter towards their edges. Of course, where there is a deep hollow in the shadow parts … you will get a darker tone. But this does not contradict the principle that generally shadows are lighter in the middle and darker towards the edges.” I tried to follow this idea, even though I don't fully understand it. I guess it means the core of a shadow is the darkest part, the core being the part just inside the edge of a shadow between the highlight area and reflected light coming from the other side. In the drawing above, that would be just inside of the vertical shadow line running from her spine at the top to the edge of her pelvis at the bottom. This is a one and a half hour drawing on plain white paper using vine charcoal in the beginning and compressed charcoal for the darkest areas, plus a charcoal pencil for details.