Linear drawing uses line to indicate an outline or a change of plane within the subject, sometimes using cross-contours to help describe the three-dimensional form. With value drawing, lines are not used except in the initial layout to place the drawing on the page. Areas of light and dark describe forms, shaded to match the fall of light and shadow across the subject. You need to be aware of your perception of the object in front of you. You know it to be a certain color or value, but we can see many variations of shade because of the fall of light. Being keenly aware of this difference between the innate surface quality and your perception of it will enable you to make the jump into realism.