GRID BOARD
A board used for fairy problem invented by W. Stead in 1953 which is divided as grid of 16 (non-overlapping) square areas each having 2×2 squares of the chessboard. Legal moves are those that cut at least one line of this grid. (Example : In Southwest corner of the grid the 2×2 area covers the squares a1-a2-b2-b1. Consider bK in a1 and wR in b1. King is not in check. Then wR moves to c1 (crossing one grid line, moving to another 2×2 area c1-c2-d2-d1) and gives check from there). (Emmanuel Manolas)