Line
Linear perspective is a graphic 
convention widely used to represent space. This system approximates many of the perceptual clues such as scale, position, and proportion that we use to recognize spatial relationships. One-point perspective is used when one face of the represented object is parallel to the front of the picture plane.  Receding horizontals called "orthogonals" converge to a single vanishing point that falls on a horizon line. Together, the horizon line and vanishing point determine the exact position of the viewer's line of sight in relation to the rendered space.