The projective plane is another surface that is commonly represented by a square with edge identifications, as in FIGURE 1c. The projective plane is the space of straight lines through the origin in 3-dimensional Euclidean space. It can be obtained from the sphere by identifying antipodal points. Like the Klein bottle, it is not orientable and can't be realized in 3-dimensional Euclidean space without self-intersections. See Barr [3], Stillwell [70] or Prasolov [58] for more information about representing surfaces. An alternative projective chess is obtained from the traditional board by adding squares at infinity [56, Chapter 6.16]. This is also called projected chess [59].