About Sudoku

Fill the 9x9 grid so that every row, every column and each of the nine 3x3 boxes contains the digits 1 to 9 exactly once. The starting clues are the only fixed points; everything else is deduced. No arithmetic is involved at any stage, which is the part most people are surprised by — a sudoku is a constraint puzzle wearing numbers as costume, and letters or colours would work identically.

Four difficulties change how much the grid gives you. Easy grids fall to scanning: find the row where only one cell can hold a 7 and place it. Harder grids withhold that, and progress comes from candidate elimination — noting what each empty cell could still be, then finding the pair of cells in a box that between them monopolise two digits, which frees every other cell in that box from both. Expert grids leave 25 clues and expect you to hold several such chains at once.

Pencil marks, undo and three hints per puzzle are built in, and the daily challenge gives everyone the same grid with a shared leaderboard. Mistakes are counted but never block you: a wrong digit shows red against its neighbours rather than stopping play.

Tips to win

• Scan for the digit with the most clues already placed — it constrains the grid hardest and often resolves several cells at once
• When scanning stalls, pencil in candidates for one box only, not the whole grid; a full sweep is slow and usually unnecessary
• Two cells in a unit that can only hold the same two digits lock that pair away from every other cell in that unit
• If you are guessing, you have missed something: every puzzle here is solvable by deduction alone