A carpender wants to install molding on the cealing of a room. He needs it to turn a 90 degree corner. In the old days the molding had a trianglar cross section and he had to cut it at a 45 degree setting angle. This was simple. Today however the molding comes in a flat section. It has to be placed on the wall at 45 degrees to the wall and 45 degrees to the cealing with a trianglar air space behind it. Now he needs it to turn a 90 degree corner. The way to do this is to cut the molding with a compound angle saw. That is the saw is angled with respect to the center line of the molding called the setting and the saw is tilted with respect to the plane of the molding. The question is how do you figure out the angle of the tilt and the angle of the blade setting. I think the answer involves the dot product. btw In the real world the saws simpley come with the angles marked on the saw.