Unlike regular sudoku, which uses digits 1 through 9 on a 9x9 grid, Hex Sudoku uses digits 0 through 9 and letters A through F on a 16x16 grid. There is exactly one of every hexadecimal digit (0-9, A-F) in every row, column, and 4x4 group. Regular sudoku has exactly 81 spaces, including prefilled ones. Every Hex Sudoku puzzle has at least 128 blank spaces for you to solve.