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. Hex Sudoku contains 100 puzzles with solutions in the back of the book.