Lesson 1 - Form Fundamentals: Cylinders
Learning to draw cylinders proficiently means becoming fluent with the rules of this geometric form. This can then translate into cross contouring organic subjects such as the human body. This is an extremely important tool in form drawing and gesture drawing - allowing efficient investigation, and simplified depiction of many of the complexities of organic form.
Cross contours also provide a way to investigate the orientation of whole masses, directly and efficiently, and so to depict the general arrangement in space of a figure. They can read quite naturally visually because natural creases in skin and other forms can resemble cross contours. You can use one or two to orient the mass, or many, subtly varied cross contours to feel out the surface. A cross contour can be as subtle and evocative of form as edges/horizons - think of the way a tan line or stripy socks show up the form.
Many of the old masters used cross contours extensively - in drawing studies, etchings and engravings and even in painting around form. This is "Directional rendering", which really means shading the form such that the lines that compose the shading are visible to some extent, and their direction is in sympathy with the form in some way. This way of rendering form continues with many contemporary fine artists and illustrators today.
How to learn to "Directionally Render"
In the hands of an expert, directional rendering can travel in any direction, often obliquely to the long axis of the form (think of a bank note drawing). But this requires that the artist build up a feel for directional rendering - an intuition for it. To do this, I think it is best to practice rendering either along or across the form so that a sense of following a line in space with an arc on a 2D surface is built up.
Click the diagrams below to enlarge