Robby-roy’s post is pretty important to learn because ellipses and cubes are building blocks for objects.
For instance, if you set a wheelbase for a car with ellipses, then you can use the box to find other proportions. If you use the cube then you can draw the ellipse etc.
Sketch-A-Day posted this link: http://userwww.sfsu.edu/trogu/420/reference/perspective/tutorials/ellipses.html