"The next essential fact always to have at the back of your mind is
"form", or the fact that the figure which you are drawing
has thickness as well as outline.
If you study closely the great masters of figure drawing
in outline, you will be amazed to discover that they
manage to indicate the "form" of a body without the
use of shading. How is this done? The secret of these
clever drawings is often in certain lines on the
figure or head and not actually on the outline — a fold,
a collar, a cuff, a crease. Look at these
lines carefully, and you will observe that they
are very correctly drawn, sometimes even more than
the actual outline. A simple illustration of this
point is in Fig. E, Plate 12."
From L. A. Doust, 
