Janet and Nick call these things “jawbreakers,” but I thought that would look a little weird as a subheading. Of the 41 examples on this page, only one breaks the pattern of two series of concentric circles: in the bottom row on the left, there is one drawing that includes three circles. (You can also see the outline of towers on the right side of that page, because Jamie was using a marker whose ink went right through the paper. I have advised him not to do that, but he is very particular about his choice of pens.)