It is obvious that if _AE_ is equal to _AC_, the whole figure will be
a cube, and each side, _aefc_ and _aegb_, will be a square in a given
vertical plane. And by making _AB_ or _AC_ longer or shorter in any
given proportion, any form of rectangle may be given to either of the
sides of the pillar. No other rule is therefore needed for drawing
squares or rectangles in vertical planes.
Also any triangle may be thus drawn in a vertical plane, by inclosing
it in a rectangle and determining, in perspective ratio, on the sides
of the rectangle, the points of their contact with the angles of the
triangle.
And if any triangle, then any polygon.