Sudoku — guidebook

Fill a nine-by-nine grid with the digits 1 to 9. Every row, column and three-by-three box must contain each digit exactly once. You never need arithmetic: the numbers are nine different symbols to place logically.

Start with a small deduction

Select an empty cell and look along its row, down its column and inside its box. Cross out the digits already present there. If only one digit remains, you have a naked single.

For example, if those three groups already contain 1, 2, 3, 4, 6, 7, 8 and 9, the selected cell must contain 5. You do not need to guess. Dark fixed clues cannot be changed; your entries appear in blue. The selected cell is highlighted, with its related cells shaded.

Two difficulty choices

Gentle puzzles can be completed using naked singles and hidden singles. A hidden single is a digit that has just one possible location within a row, column or box, even if that cell has several candidates.

Tricky puzzles require more than those two techniques. There is no promised upper difficulty ceiling. You can use notes, Undo and Hint without a score penalty or time limit.

This edition contains six original base puzzles, three in each grade. Digits, rows, columns and orientation are transformed to produce different-looking valid boards. These are variations on that finite collection, not a promise of unlimited structurally different puzzles. Every puzzle has one solution.

Controls

• Touch or click a cell, then a number on the keypad.
• With the grid focused, arrow keys move the selected cell and number keys 1–9 enter digits.
• Backspace, Delete or Erase clears your entry and any notes in the cell.
• Notes toggles pencil mode. Undo reverses the last edit. Hint offers help.
• New puzzle replaces the current puzzle at the same difficulty. Changing Difficulty starts a puzzle of that grade. Both ask before discarding unfinished edited work.

The board waits for you indefinitely; no pause button is needed. On a small screen, scroll down for the keypad and help.

Pencil notes

With Notes on, choosing a digit adds or removes that small candidate mark. Notes are your own working list: they are not automatically calculated or updated when another cell changes. A completed entry must be erased before notes can be added there.

A useful habit is to note candidates only where they help a decision, then tidy them when you place a related digit. Turning Notes off returns to normal entry. Placing a normal digit clears that cell's notes.

Red marks and hints

Red marks identify duplicates within a row, column or box. They do not prove every other entry is correct: a wrong digit can avoid a duplicate for a while and still make the final solution impossible.

Hint checks actual entries, not your pencil notes. It works in this order:

1. If an entry disagrees with the unique solution, it clears one such entry and explains where it was.
2. Otherwise it places a naked or hidden single and explains the deduction.
3. If neither single is available, it explicitly labels the help as a solution-assisted reveal. This uses the stored answer; it is not pretending to demonstrate a harder logical technique.

Hints are undoable. If you want to solve independently, you can ignore Hint and use only the board, notes and duplicate marks.

A beginner's playbook

Scan nearly complete rows, columns and boxes first. A group missing only one digit is an easy starting point. After placing a number, revisit the three groups it touches: that new information may force another cell.

Also scan one digit at a time. If 7 is blocked from every position in a box except one, that remaining position must be 7. Alternate this search with single-cell candidate checks. When stuck, recheck notes and recent entries before making an unsupported guess.

For Tricky puzzles, notes may expose pairs or interactions between a box and a row or column. Hint is available when you would rather move forward than pursue a difficult technique; the game makes its level of assistance clear.

Undo, completion and replay

Undo restores up to 100 previous edits, including number entries, erased cells, notes and hints. Filling all 81 cells with the unique correct solution finishes the puzzle. The completion message appears and normal entry controls stop. You may still Undo to revisit an earlier state or start a new puzzle.

There is no timer, leaderboard, streak or online competition in this edition. Take as long as you like.

Saves, installation and offline play

The puzzle, entries, notes and undo history save locally in this browser. Reloading restores them; selection returns to the first cell and Notes mode starts off. Saves do not transfer between browsers, devices or website addresses. Clearing browser data removes them.

Load the complete standalone game online once. Install uses the browser's Install app or Add to Home Screen option where supported; in iPhone/iPad Safari use Share, then Add to Home Screen. Installation is optional. The game and guidebook work offline after the complete first online load. An embedded game leaves installation to the host site.

The full guidebook opens in a separate tab to preserve the puzzle. Your browser can print it or save a PDF. Physical-phone installation/performance has not been verified on a device.

Troubleshooting

If a digit will not change, it may be a fixed clue or the puzzle may be complete. If small digits appear, turn Notes off. If a new note is refused, erase the existing normal entry first. Red duplicates require checking both the new number and related entries.

If saved work is missing, verify the same browser and website address and check storage settings. After clearing offline data, load the page online again.