The proof that the m positions in C are nonattacking is perhaps best done by con sidering three separate cases: (a) m divisible by 6, (b) m divisible by 3 but not by 6, and (c) m not divisible by 3. We briefly describe case (a). From the construction of C, no two positions are in the same column. So we must show that the positions are also in distinct rows, distinct negative diagonals, and distinct positive diagonals. This gives three things to check: