Normal sudoku rules apply. The sum of two orthogonal neighbours is at most 13 (ie no domino in the grid can sum to more than 13). The sum of two 'point symmetric' cells is always 10. (Point symmetric cells are pairs that are mirror images of each other across the exact centre of the 9x9 grid (the middle of the central 3x3 box). ie Rotating the grid 180° around this central point maps a cell directly onto its symmetric partner). So, for example, r7c1's symmetric counterpart is r3c9.