Refers to the contrast in a picture, the range of tones from light to dark. Experienced artists gain great subtlety in manipulating tones to produce an illusion of actual light emanating from a picture. This skill does not come easily to those learning to paint. Painting just in black and white doesn’t seem very exciting but it is the best method for discovery in this area.
The old masters habitually made tonal studies in preparation for a painting. This was a wise approach, it being far easier to see the underlying tonal composition of a work when in just black and white. The tonal composition is one of the most important elements in successful painting.