Underpainting the Canvas

Before an artist starts the actual painting, he may underpaint or tone the canvas. Think of spreading a light color (thinned paint) over the canvas, not evenly, but just to soften the harsh whiteness of the canvas. Some artists, do a sketch or charcoal or thin paint outline of the figure they plan to paint. This may be on top of an underpainting or an artist might not use underpainting at all.

Resources

Underpainting the Canvas – a How To
"Art Glossary: Underpainting" – a definition
“Palettes and Techniques of the Old Masters: Leonardo da Vinci” – discusses how he used underpainting.