The grill then acts as a guide for directions of the lines.

First of all, select a suitable eye level (horizon) line, and then start drawing freehand. Set the vanishing points. Then contstruct a grill on the left, divided into many equal segments. When perspective lines are drawn from each of these segments across the page, it will be seen that the corresponding segments on the right-hand grid are much shorter. The vanishing point on the right is beyond the margin of the picture.