Each column of nine cells must contain each of the numbers from 1 to 9 once and only once. Each of the nine 3 by 3 boxes of nine cells must contain each of the numbers from 1 to 9 once and only once. the sudoku starts with a partially filled grid and you must complete the grid while following the rules above. If you complete the grid, you have solved the sudoku. Each sudoku on this site has one and only one unique solution.