Mass is the volumetric form of an object. As was mentioned in the discussion of hatching and cross-hatching, it is necessary for artists to develop the interior definition of objects in order for the viewer to recognize that such objects exist in real three-dimensional space, the space we ourselves occupy. For example, a human face is not as flat as a pancake, and therefore should be depicted, as in Michelangelo’s Head of Satyr, with hills and valleys simulated by a careful distribution of light areas and dark areas.