The notched cylinder is made in two halves which are initially glued together with a sheet of newspaper in the joint. After turning, the two halved can easily be separated. The cross section of the notched cylinder is a square with a small rectangle removed from each edge When this cross-section is rotated through 90 degrees it has the same shape. So the two halves can be re-joined in a different way This is a simple example of the "Streptohedron" shapes described by David Springett in his book "Adventures in Woodturning". Ian Stewart described a similar project which he called a "Sphericon" in "Mathematical Games", Scientific American, Oct 1999