Hold a box in your hand in such a position that you only see the front and top of it. Observe that as we view the top, its outer edges appear to converge. This condition is caused by an optical illusion. As the top turns away from us, the relationship between the length and width changes or diminishes. This phenomenon is known as foreshortening. Observe also that as we look at the face of the box, it is seen in its true form. The reason for this is that it is perpendicular to our line of vision.