Perspective is what lets us control the spatial elements of our drawing-what makes visual realism, or illusionism, look "right." You have already been introduced to the perspective of the cube. We now explore linear perspective in greater depth. And depth is what linear perspective is about. When you understand it, you can confidently establish the illusion of depth in your pictures.
Railroad tracks appear to get closer together as they recede in space toward the horizon. The point where they appear to meet is called the vanishing point.