The first two shells are quite unusual. In the 3 shell, The 3 shell, I stop using a circle to represent the path of the samples and use straight lines to connect the samples instead, so shells become polygons (as the number of samples increases these polygons converge to a circle). Note how the exclusion lines are building up. They extend to infinity, but this is impractical to show in the diagram. With the 3 shell in place it is now easy to show how the diagram works, by explaining the simple rules and demonstrating them on the diagram. We can then work back to see if these rules apply to the 1 and 2 shells, and work forward to see if they work elsewhere too.