Final comments
We finish with two comments. First, it would be interesting to extend Theorems 1 and 2 to pillow chessboards that do not have the usual black-white pattern, as is already the case for the results on the torus. This means considering pillow boards that are obtained from the n x m torus, with m and/or n odd, by identifying diametrically opposite squares, as discussed above. This entails some unusual features; when n and m are both odd, the fundamental domain no longer consists of an integer number of squares, so the "board" looks rather odd. Also, if m is odd and n is even, one of the squares has an edge which is identified with itself!
Second, we remark that pillow chess is by no means the first chess game to be played on the sphere. There is a great number of chess variants (see Pritchard [59] and the web site www.chessvariants.com). In particular, we mention Global Chess, which is a commercial variant played on two revolving disks representing the hemispheres (here the "squares" at the poles are triangles), and Andrea Mori's small spherical chess, which is obtained from a cylindrical board by adding two additional (pole) squares, one at each end. Don Miller's spherical chess [59] as modified by Leo Nadvomey, is very close in appearance to pillow chess (the identifications are shown in FIGURE 16), but in fact this board is not a sphere but a Klein bottle!