Regular Sudoku has a 9x9 grid with one of every digit 1-9 in every row, column, and 3x3 group. WTF Sudoku has a 25x25 grid with one of every letter A-Y in every row, column and 5x5 group. Regular Sudoku has exactly 81 spaces, including pre-filled ones. WTF Sudoku has at least 256 blank spaces for you to complete.