Rule 9
The farther a point is removed from the picture plane the nearer does it appear to approach the horizon, so long as it is viewed from the same position.
Rule 10
Horizontals in the same plane which are drawn to the same point on the horizon are perspectively parallel to each other.
BOOK SECOND
[THE PRACTICE OF PERSPECTIVE]
In the foregoing book we have explained the theory or science of perspective; we now have to make use of our knowledge and to apply it to the drawing of figures and the various objects that we wish to depict.
The first of these will be a square with two of its sides parallel to the picture plane and the other two at right angles to it, and which we call