An artist's handling of edges is one of the drawing basics and of great importance if a drawing is to be convincing.
Contour lines are a useful lie a draftsman uses to indicate the edge of a form in a line drawing. In truth, we don’t see a line marking the edge of a face, we merely see where the form curves away from view. Drawing a solid line on the edge of elements suggests shapes, not forms—a draftsman must take care to imply the other planes not visible from the viewer’s vantage point. Plus, simply concentrating on the contour lines can distract an artist from the important task of portraying the gesture of the model, which usually radiates from the interior of a figure. For this and for other practical reasons, an artist’s handling of edges is of great importance if a drawing is to be convincing.
Curves are hard to accurately render. Many drawing instructors recommend using only straight lines for edges, softening them into curves where necessary later. If you think this is a beginner’s crutch, consider how Rubens, a master draftsman, used this method.
Edges do much of the work in suggesting depth. A thick line brings the shape forward; Dan Gheno’s drawing reminds us that a light, thin line indicates a plane receding into the background. But edges aren’t just about lines. In more tonal pieces, a harder edge and a marked contrast between planes create a form that is closer to the viewer than one with a softer, lighter look. This is essential for cast shadows—a shadow is sharpest at the point where it touches the object casting it, and it diffuses as the shadow lengthens away from the object.
In his book Mastering Drawing the Human Figure From Life, Memory, Imagination, (self-published), Jack Faragasso points out that one should always be aware that the most important edges are those that indicate light and dark patterns. He uses as an example the thick ruffled collars that often appear in Rembrandt’s paintings—the important edges in such collars are not the individual twists and folds, but rather the edge of the shadow as the collar moves into the light. Effectively depicting that line will do more to make the ruff convincing than a hundred detailed lines.