First, let me apologize for yesterday's poor image. I re-read the post later in the day and realized how bad the image was. Here's a view of the inside of the door, where the construction details become evident. The door's racking resistance comes from the interior frame, made of two vertical stiles, three horizontal rails and two braces. If you remember, the rail to stile joinery is epoxied threaded rod. Together with the braces, the frame is extremely rigid and will never sag. Also, by gluing the first and fifth exterior boards to their respective stiles, the net stile thickness is around 1.5" of very stable material. The gist: this is a very lightweight, strong and stable door. Oh yeah, it looks pretty good too (or at least better than the door it's covering up).