Sudoku — Play Free Online

The rules, and the only thing they imply

Nine rows, nine columns, nine three-by-three boxes. Each has to end up containing the digits 1 to 9 exactly once. Nothing else is a rule, and everything you will ever do in a Sudoku follows from those three constraints intersecting.

A correctly made puzzle has exactly one solution and can always be reached by deduction. If you are guessing, you have missed something — which is good news, because it means the information is on the board.

The two techniques that solve most puzzles

• Naked single: a cell where eight of the nine digits already appear in its row, column or box. Only one digit fits, so write it.
• Hidden single: a digit that can only go in one cell of a row, column or box, even though that cell could in principle hold other digits. Scan digit by digit rather than cell by cell to find these.
• Between them these two clear every easy and most medium puzzles. Learn to spot hidden singles and your solving speed roughly doubles.

Pencil marks, used properly

Once scanning stops producing anything, fill in candidates — the small digits still possible in each empty cell. The point is not to write them and stare; it is to look for cells with only two candidates, and for a candidate that appears only twice in a row or box.

A pair of cells in the same unit holding the same two candidates removes those digits from every other cell in that unit. That single pattern, called a naked pair, is usually what unlocks a hard puzzle.

How to get faster

Stop solving in reading order. Work the digit that appears most often on the board first, because it has the most constraints and therefore the most forced placements. Then move to the next most common.

And resist filling pencil marks too early. On an easy puzzle they cost more time than they save, and they clutter the board you are trying to read.