Let's talk about figure drawing. In most drawing methods, the first task is to create a line drawing of the edges of all significant forms. In the drawing system I've been studying, this line drawing is called the contour. Each part of the contour represents a horizon. In other words, it is the last part of the form you see before it curves out of sight.
In developing an acurate contour, it's helpful not only to learn to draw what you see, but also to understand certain principles by which organic form is organized. Without that understanding, it's easy to unconsciously impose the wrong kind of order on the form you're looking at, and then fail to see the small errors that creep in. That's why it's useful to have a checklist in your head of how organic forms are organized, so that you can make sure what you think you see is what you really see.
That's not to say that you should impose some kind of predefined idea of what the form should look like. But the fact is that every organic form I've ever drawn follows these principles, so understanding them helps you to be an informed observer. For example, a couple of weeks ago my teacher looked at a drawing I was working on and noted that the terminator (the shadow edge) of a forearm was parallel to the contour of the top of the arm. I'll talk about the avoidance of parallel forms later, but for now just understand that I looked at the arm in question and realized that I had imposed the wrong kind of order on my drawing. The terminator on the form I was looking at was not parallel to the edge of the form, but I had drawn it that way. Keeping these kinds of principles in mind makes it easier to draw what you see, because it helps you draw correctly and to identify and fix the mistakes that you make.
This will be an ongoing series of posts to describe a series of principles that are useful to understand in drawing the contour. These principles are at least as important has having a technical knowledge of anatomy, because they apply to all of the forms of the body, and do not tend to vary from person to person so much as formal anatomy does.
CURVES ARE CONVEX, NOT CONCAVE. All forms of the body are composed of curves, not straight lines. While it is quite possible to represent the contour via a series of straight line segments (and, in fact, many drawing methods start with an initial block-in that consists of straight lines) the actual contour consists of curves. Some of those curves are relatively straight, but they will look wrong unless you find the real curve rather than simplifying them into a straight line.
And the curves are not just any curve; they are always convex (pushing outward). That is the case even if the model is very skinny. The forms of the body are full, not hollow. Of course, there are many parts of the body where the largest forms go inward. Think of the web between the thumb and the first finger, for example. But if you look closely, the smaller curves that make up these larger forms are full; sometimes very subtly so. They are composed of convex curves smoothly joined together. If you draw these forms as concave (hollow), as they may at first appear, they won't look quite right.
If you think about it, this principle makes sense. The body is a mass of tissue enclosed by skin. The tissue (muscle, bone, sinew, fat) pushes outward against other tissue and against the skin. That tension creates full, outward-pressing forms. Where the skin is slack, such as around wrinkes or where the skin is larger than the tissue it encloses (as in parts of the body of an older person), the skin still tends to fall in curves that are convex, because the interconnecting tissue that the skin itself is composed of tends to push outward against itself.
Try looking at your own body, or that of people around you (they don't have to take off their clothes, although that can be nice). Look for the way that every horizon of every part of the body is composed of interconnected convex curves.
More on this later.