If your pencil strokes show through on your final drawing, they can help suggest the surface planes of your subject, adding depth and dimension. Energetic and loose pencil strokes are dynamic and interesting. I like to see an element of the hand of the artist in the final drawing. Experiment with bold linework but do not use it as a coverup for inaccurate observation. One way of using line direction to show the orientation of the planes of your subject is to make your shading lines in the same direction that water would flow if it were on the surface of the object. The direction of these lines change when you come to a new plane, except in the case of vertical planes (all vertical planes have vertical lines as in the cube). A change in plane also means a change in value (dark to light). The more abrupt the change in value, the sharper the edge between two planes. For more on this, see How to Draw Rocks. You can also use linework to suggest a sphere. Use latitude or longitude lines to suggest a spherical shape. Study the way the ends of these lines tuck around the back side of the sphere. On this mountain lion skull, notice how the direction of the linework and changes in shading value suggest changes in the planes of the skull. Click on the image to enlarge. Notice how I use these lines on the lower jaw and the zygomatic arch (cheek bone). Click on the first image below to start a step-by-step slideshow of my process drawing a weathered bobcat skull. Notice how bold linework adds interest and suggests the planes of the skull.