Fewer direct hints and a greater need for systematic candidate tracking to avoid guessing.
9×9 Sudoku board. Use the arrow keys to move between cells; Home and End go to the start or end of a row; Ctrl or Command + Home/End goes to the first/last cell on the board. Keys 1–9 enter a digit in the current mode; Shift + 1–9 adds a quick Note without turning Notes on; 0, Delete, or the numpad decimal key clears a cell; N toggles Notes, A toggles Auto Candidates, and Space pauses the game.
Levels 1–2 only guide you and do not use a hint; level 3 applies the solving step and uses 1 hint.
Select an empty cell to see valid candidates.
Auto candidates and Notes mode are remembered separately for Classic and Killer. Pause state and game progress are not synced as preferences.
Hard requires careful candidate maintenance. When there are no singles, look for naked pairs, hidden pairs, and candidates locked into the same row or column within a block.
Difficulty is not assigned from clue count alone. The site grader follows human techniques first, records the hardest technique reached, and uses residual search only when implemented logical techniques stall.
Techniques found: Naked Single · Hidden Single · Locked Candidates
This is one representative verified puzzle from the current verified quality pool. Difficulty remains an estimate because player experience changes how hard the same board feels.
Hard Sudoku is for players who already use candidates comfortably and no longer depend on obvious singles. Many games require you to spot naked pairs, hidden pairs, or locked candidates before the next move appears.
At this level, accuracy matters more than speed. One incorrect note can make the whole later deduction chain difficult to control, so update candidates immediately after every certain elimination.
Hard Sudoku commonly rewards accurate candidates, naked pairs, hidden pairs and locked candidates. These techniques often create the single that opens the next part of the grid.
No. Guessing makes it difficult to verify the logic and can hide an earlier candidate mistake. Work through candidates systematically and move to another region when one area is stuck.
Hard puzzles usually rely more heavily on pairs and locked candidates, while Expert puzzles more often require interactions across several regions and advanced patterns such as triples or X-Wing.
The game board supports Notes, Undo, Pause, Hints, and saved progress. You can sign in later if you want to sync your achievements.
