If you wish to thoroughly accustom yourself to correct and good positions for your fingers, fasten a frame or a loom divided into squares by threads between your eye and the nude figure which you are representing, and then make the same squares upon the paper where you wish to draw the said nude but very faintly. You should then put a pellet of wax on a part of the network to serve as a mark which as you look at your model should always cover the pit of the throat, or if he should have turned his back make it cover one of the vertebrae of the neck.... The squares you draw may be as much smaller than those of the network in proportion as you wish your figure to be less than life size...

The Notebooks of Leonardo Da Vinci (MacCurdy, 1938) - XXIX Precepts of the Painter