One of the basics, by the same greatest, techniques calls Hatching. It helps to understand every part of the drawing that grows over paper. An interesting way of visual representation.

Hatching (hachure in French) is an artistic technique used to create tonal or shading effects by drawing (or painting or scribing) closely spaced parallel lines. (It is also used in monochromatic heraldic representations to indicate what the tincture of a "full-colour" emblazon would be.)