Who would have thought that, in order to draw an open door in correct perspective, it would be necessary to draw so many guidelines first:

To figure out the way the door opens, you have to draw an ellipse to describe the path the outside edge of the door swings.  Before that, it’s necessary to draw an accurate square in perspective on the floor that contains the ellipse.  And before that, you’ve drawn a triangle from convergence lines originating at the vanishing point.  A lot of geometry.

Luckily, geometry was the one math class I did enjoy in school!  And on the heels of the much more subjective, interpretive, nebulous work of shading, it’s such a refreshing change to depend on math to figure out your drawing.  It’s comforting– relaxing, even– to know that, as long as you use your ruler right, you’re going to get the drawing right!