Each cell in the central 3x3 box maps to a box in the larger grid, and shares parity (odd/even) with the winning tic-tac-toe line in that box. No 3x3 box can have both a line of three even digits and one of three odd digits.
Each cell in the central 3x3 box maps to a box in the larger grid, and shares parity (odd/even) with the winning tic-tac-toe line in that box. No 3x3 box can have both a line of three even digits and one of three odd digits.