Fig. 27.
[Rule 5]
All horizontals which are at right angles to the picture plane are drawn to the point of sight.
Thus the lines AB and CD (Fig. 28) are horizontal or parallel to the ground plane, and are also at right angles to the picture plane K. It will be seen that the perspective lines Ba·, Dc·, must, according to the laws of projection, be drawn to the point of sight.
Fig. 28.
This is the most important rule in perspective (see [Fig. 7] at beginning of Definitions).