Because paper is white, the substance of drawing is shadow.
Shadow is what we look for when we draw. Once we establish the outline, we begin to shade the drawing. Only at the end of the process are the illuminated areas revealed.
The drawing at the left is a 20-minute study of a costumed model with a brush and black ink.
What if we could reverse that thinking and explore the structure of the light masses from the beginning? The second study uses brown paper and white gouache to define the areas touched by light. After a quick line drawing, I painted in the light shapes, in this case ignoring or downplaying variations in the light.
The shadow side of the form had to take care of itself, unless I placed a patch of light behind the model to define the shadow side contour.
This is a very good exercise for any painter or any student of composition, because the light masses should really concern you more than the placement of darks.