Students draw a horizontal line for their horizon line, before drawing shapes (geometric or organic) above and below the line. We discuss the difference between shape (2D) and form (3D) and when I name a shape, they tell me the form version. A circle can become a sphere or a cylinder. A triangle can become a pyramid or a triangular prism. In order to add dimension to their flat shapes they need to pick a point on the horizon line. This is called the vanishing point. If you watch a cowboy ride off into the sunset, he will appear smaller and smaller until he finally disappears (thanks to the curvature of the earth). This is the vanishing point. Poet laureate, Billy Collins was referring to the vanishing point when he wrote, "You have heard of the apple that astonished Paris? This is the nostril of the ant that inhaled the universe."
Rulers were used to connect the vanishing point to the corners of shapes. If there are curves, the line becomes like a tangent, going to the outer edge of the shape. Finally, value and color can be added. Most of my students who did the exercise are visually impaired, but the example here is a student who is completely blind. He may never experience watching a cowboy ride off into the sunset, but he can understand the concept better now that he's made a tactile graphic using the principles of linear perspective. And this concept helps those of us with sight understand why things look the way they do.