Sudoku

The rules. Fill the 9×9 grid so every row, every column, and every 3×3 box contains the digits 1 through 9 with no repeats. The givens (the bold black digits) are fixed; you fill in the rest.

Placing a digit. Click or tap any empty cell, then either click a digit on the palette or press a number key (1–9) on the keyboard. Use Backspace or Delete (or the Erase button) to clear the cell.

Pencil marks. Sudoku players track candidate digits in cells with small notes. Press N (or click Notes) to toggle notes mode — in notes mode, clicking a digit toggles it as a candidate inside the selected cell rather than placing it. To enter a single note without switching modes, hold Shift while pressing the digit. Placing a final digit clears all notes from that cell and from the digit's row, column, and box automatically.

Conflict highlighting. If you place a digit that already appears in the same row, column, or box, both cells turn rose-pink. The page does not tell you when a digit is "wrong but consistent" — that's the puzzle's job. Hit Hint if you get stuck.

Hint. The Hint button highlights the next deducible cell and explains the technique that uncovers it — for example, "Naked single — only 8 fits in row 4, column 7". Hint never places the digit for you. If your board has an incorrect entry that's preventing further deduction, Hint will say so.

Difficulty. Difficulty is graded by the solving techniques each puzzle requires. Easy needs only naked singles; Medium adds hidden singles; Hard adds naked pairs, hidden pairs, and pointing pairs; Expert adds the X-wing pattern. The clue count varies; the technique requirement is what defines each level.

Keyboard shortcuts. Arrow keys move the selection. 1–9 place a digit (or toggle a note in notes mode). Shift+1–9 always toggles a note. N toggles notes mode. Backspace / Delete clears the cell. H requests a hint. P pauses or resumes the timer.