Overlapping and Gradations
Add Form and Depth

Three-dimensional space and form are made visible in painting chiefly through the overlapping of forms and through the play of light and shadow across the scene. The reason light and shadow (depending on the direction and intensity of the light) help to make forms look three-dimensional is mainly because they introduce gradation. Using gradated tones on forms can help them read as three-dimensional.

This is a line drawing of an orange on a table. This one circular line by itself is not enough to give any real information about where it exists in three-dimensional space...