OF THE BACKGROUND THAT THE PAINTER SHOULD ADOPT IN HIS WORKS.
Since experience shows us that all bodies are surrounded by light and shade it is necessary that you, O Painter, should so arrange that the side which is in light shall terminate against a dark body and likewise that the shadow side shall terminate against a light body. And by [following] this rule you will add greatly to the relief of your figures.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.