About the game
Sudoku Online hands you a 9×9 grid with some digits already placed. Fill the rest so that 1 through 9 each show up once in every row, every column and every 3×3 box. Easy through Expert set how many clues you start with.
Pick a cell whose row, column and box are already crowded, since that leaves the fewest digits to choose between. Notes mode keeps several candidates in one square so you can cross them off later. X-Sudoku, Windoku, Killer and Kropki bend the rules with diagonals, extra regions, target sums and dot markers. Every grid has a single answer you can reach by thinking.