In this tutorial we work with both pencil and pen, to create a more refined, contrasted drawing of a rib cage. We want to have a good reference image to guide us throughout the shading process of the ribcage. What we want to pay attention to the most is how we define the background and the foreground of the ribcage. We can do this by playing around with different tonal variations that will bring the front row of ribs that stand in the foreground and the back row of ribs that sit in the background. A good suggestion is to darken the ribs that sit in the background, as this will make them seem more shaded as they are covered and less exposed to a light source. Whereas, the front row of ribs can have a lighter tonal value as they sit in the foreground, as they are more exposed to the light source. Another good suggestion, is lightly layer marks over each other, by doing so you add contrast in a more controlled fashion. You do this by constantly referring to a source image, that should guide your shading process, helping you draw marks more accurately.