Drawing in Upright Position

I know lots of artists like to lay a drawing flat when they work on it, and I used to do the same.  I have long since switched to placing my drawing surface on my easel attached to a strong board for stability.  If I draw on a flat surface, there is a problem with foreshortening I don't see until I stand the drawing up and view it in a vertical position.  Its as if the figure I am drawing was seen horizontal and all the body parts were foreshortened accordingly.  I think I will do a demo of this shortly so that I can illustrate to my students what I am talking about.