Here is a drawing by Pierre-Paul Prodhon, a French artist who lived from 1758 until 1823. The image is courtesy of www.artrenewal.org . whose online museum is something you should be aware of, as it is a great resource. I am routinely indebted to them for images without which much of this tutorial would not be possible.
This beautiful figure is a good example in which to clearly observe an artist finding the shadow edge or our "bed bug line". His shadow edge is easy to see and the two worlds, the light and the shadow are carefully divided. I am now going to write something you will have to read a few times to understand. The first time I heard it, I thought it was gibberish;
THE DARKEST DARK IN THE LIGHTS IS ALWAYS LIGHTER THAN THE LIGHTEST LIGHT IN THE DARKS.
THE LIGHTEST LIGHT IN THE DARKS IS ALWAYS DARKER THAN THE DARKEST DARK IN THE LIGHTS.
There are two separate worlds with no note occurring in both. When we draw, we are "sorting " the values into the two great camps, light and shadow. Everything belongs to one or the other. Either the light strikes it or it doesn't.
No matter how dark some gradation in the halftone may appear, it is never as low in value as anything on the shadow side of our drawing. When you get this overly dark halftone it is because you are comparing one part of the halftone against another, rather than looking at the object in its entirety. This is what causes a problem called over modeling. More on that in the future.
Routinely when I teach I will explain this to students who "get" the concept. Then when I join them at their easel, I will place the heel of my brush at some random point on their canvas and ask them "is this in the light, or is this in the shadow?" They are sort of dumbfounded and will look down at their feet and say they don't know, or that its sort of in both, or neither. Every time your brush or pencil touches that canvas you need to know whether that mark you are making is in the light or if it is in the shadow. Every time. If you place one note from the light into the shadow the illusion of form vanishes.
PS. the reason I am finishing the aforementioned charcoal drawing from left to right is, because that way ( since I am right handed) the heel of my hand is to the right of, and not sitting on and smearing the delicate area of drawing I have just completed.