 Note that the 3D-surfaces above enclose 4D-hypervolumes, which are the 4-cones proper.The spherical cone consists of two unbounded "nappes", which meet at the origin and are the analogues of the nappes of the 3-dimensional conical surface.The "upper nappe" corresponds with the half with positive "w"-coordinates, and the "lower nappe" corresponds with the half with negative "w"-coordinates.