Hard Sudoku — Play Free Online

On a hard Sudoku the singles run out early, and the game changes character: instead of asking "what goes here?", you'll spend most of the solve asking "what can't go here?". Progress comes from eliminating candidates until a placement falls out — which makes complete, tidy pencil marks close to mandatory.

The key skills are the pair techniques. A naked pair — two cells in a unit sharing the same two candidates — wipes those digits from the rest of the unit; hidden pairs do the same job from the opposite direction. Chain a few eliminations together and positions that looked stuck come apart.

Hard is also where solving discipline starts to matter as much as technique. Update your pencil marks after every placement, or yesterday's candidates will quietly poison today's deductions. Most "impossible" hard puzzles are really bookkeeping errors — the logic was fine; the notes weren't.