Because of the way radial projection works, the projection of two parallel lines in 3D will appear to converge, or "vanish", towards the same point on the picture plane. These points are called Vanishing Points (VP)

VP Facts

• The position of the lines does not matter, the vanishing point changes only with their orientation.
• There are actually two Vanishing Points for each edge, but you don't always see them. (One is on the front half of the Picture Sphere, the other is on the back half.)
• Lines parallel to the viewing direction converge to the Principal Point.
• Rotating a line that goes forward to the right, will move its vanishing point to the right of the principal point, however, note that the vanishing point will remain on the horizon line. Which brings the following:
• A line parallel with the horizontal plane will have its vanishing point sitting on the horizon line.
• A line parallel with the vertical plane will have its vanishing point sitting on the median line.
• A line parallel with the transversal plane will have its vanishing point sitting on the transversal line.