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Shading is the massing of marks to create an illusion of dimensionality. Gradually darkening the surface gives the impression that light is falling on the form. If highlights and shadows are added in a way that indicates a consistent light source and casts shadows, then a strong sense of dimensionality can be achieved.  Shading can be done in a variety of media and techniques that can drastically alter the texture and character of the forms. This expressive use of shading can have a strong effect on the emotional impact of the drawing.