Sudoku Rules

Sudoku is one of the most famous logic puzzles in the world. Despite its Japanese name (from sūji wa dokushin ni kagiru, "the digits must remain single"), its origins go back to the Latin squares studied by the Swiss mathematician Euler in the 18th century; the modern form was born in the United States in the 1970s and later became a worldwide phenomenon through Japan.

No arithmetic and no luck are required: Sudoku is solved purely by logic. Every puzzle in this online version has a guaranteed unique solution, so you will never have to guess.

This game is constantly evolving: new features and improvements are added regularly. If you find something that doesn't work as expected or have suggestions, write to us at postmaster@lucianomanenti.com — your feedback is precious!

1. Goal of the Game

The grid is made of 81 cells, organized in 9 rows, 9 columns and 9 3×3 boxes. Some cells are already filled in (the clues). The goal is to fill every empty cell so that:

every row contains all the numbers from 1 to 9, with no repetitions;
every column contains all the numbers from 1 to 9, with no repetitions;
every 3×3 box contains all the numbers from 1 to 9, with no repetitions.

That's all: three simple constraints, from which a surprising depth arises.

2. Solving Techniques

Hidden Single (where can the 7 go?)

Pick a number and a box: if there is only one cell in that box where the number can go (because the other cells are blocked by rows and columns that already contain it), then it must go there. This is the most common technique and is often enough on its own for easy puzzles.

Naked Single (what can go here?)

Pick an empty cell and list the numbers that appear neither in its row, nor in its column, nor in its box. If only one candidate remains, you have found the value of that cell.

Pairs and Candidates (for hard puzzles)

When singles are not enough, the pencil comes into play: note down the possible numbers in each cell. If two cells in the same row (or column, or box) have exactly the same two candidates, those two numbers must go in those two cells: you can remove them as candidates from every other cell of the group. This "naked pair" often unlocks a chain of deductions.

A small example

Look at this box together with its top row:

5 . . | 1 2 3 . 8 . | . . 9 |

The number 1 is missing (among others) from the left box. It cannot go in the first row of the box, because that row already contains a 1 further right. That leaves the two free cells in the rows below: if one of those columns also contains a 1 already, the cell for the number 1 is determined. Reasoning like this, repeated, leads all the way to the complete solution.

3. Difficulty Levels

Level	Starting clues	Recommended techniques
Easy	40 of 81	Hidden and naked singles are always enough.
Medium	32 of 81	Hidden singles alone are never enough: pencil notes and some patience are needed too.
Hard	26 of 81	Candidate pairs, chained deductions and sometimes advanced techniques.

Easy and Medium puzzles are generated so that they can always be fully solved with simple logical deductions (singles and candidate pairs), without ever having to guess. At the Hard level this guarantee does not hold: more advanced techniques may be required.

How to play "Sudoku Luciano"

Select an empty cell: click an empty cell in the grid. Its row, column and box are highlighted to help you find your bearings.
Armed number: click a number button on the right, a 1-9 key on your keyboard, or a cell that already holds a number, to "arm" that number: the button lights up and the number stays attached to your mouse cursor. With a number armed, every cell holding it turns dark green, and every row, column and box touched by those cells turns light green — so you can spot at a glance the empty cells where that number could go; cells already filled with a different number that fall outside those rows/columns/boxes turn grey instead, to stand out visually from the rest. Clicking the same number (or the same cell) again disarms it. If you armed the number by clicking a cell that already held it, that specific cell stays highlighted in yellow: it's the target of the Erase button, which in this mode clears exactly that cell (unless it's a starting cell). Each button shows how many copies of that number are still missing.
Enter a number: with a number armed, click an empty cell to place it right away (it stays armed for further placements). Alternatively, select an empty cell first and then use the number buttons or the 1-9 keys to fill it. If the number you just entered is flagged as a mistake and you click that very same cell again right after, it gets removed (a targeted "undo" for the wrong placement); if it's correct instead, or you click a different cell holding the same number, that cell just arms normally without erasing anything.
Mistakes: numbers flagged as wrong are shown in red and counted in the top panel. You can fix them by overwriting or erasing them. Two options below the buttons (remembered by the browser) control the flagging: Highlight obvious mistakes (on by default) only flags conflicts with the numbers already visible in the row, column or box; Highlight all mistakes (off by default) checks every number against the solution. Turn both off to play "the old-fashioned way", with no help at all. Every flagged mistake carries a time penalty: 10 seconds if the number conflicts with numbers already visible (the information to avoid it was right in front of you), 30 seconds if the mistake only shows up against the solution.
✏️ Pencil: toggle note mode (also with the N key) to jot down the possible candidates in a cell; a little pencil stays attached to the cursor while it's on. Turning it on automatically disarms any armed number (and vice versa: arming a number turns pencil mode off), since the two are alternative placement modes. When you enter a final number, notes for that number in the connected cells are cleaned up automatically.
❌ Erase: clear the selected cell, or the cell highlighted in yellow while a number is armed (also with Del or Backspace).
↶ Undo: undo the last move, notes included (also with Ctrl+Z).
💡 Hint: a "didactic" helper. Pressing the button highlights a cell that can be logically deduced from the visible numbers and opens a panel with three choices: Explanation shows the reasoning behind the deduction (and therefore the number), Fill it in fills the cell, Close dismisses the hint without changing anything. If the grid contains mistakes, the hint points those out first, and the panel offers to erase the wrong number. Only when no simple deduction exists (it can happen at the Hard level) does the hint point to the cell with the fewest candidates and offer to reveal it. Only actual fills are counted in the final summary. A hint carries a time penalty: opening the cell (the highlight) costs 10 seconds; asking for the Explanation or choosing Fill it in adds another 20 seconds, only once even if you use both. Closing right away with nothing else leaves just the 10 seconds for opening; erasing a mistake that is already on the grid adds no penalty (the mistake is already counted when it was entered).
Arrow keys: move the selection around the grid.

Control Buttons

NEW: abandon the current puzzle (after confirmation) and open the difficulty selection panel.
HOW TO PLAY: open a quick reminder of the rules directly in the game.
🔊 / 🔇 (Audio): enable or disable sound effects. Your preference is global and will be remembered by the browser on every page of the site.

Automatic Saving

The game is saved automatically in your browser after every move: if you close the page or your computer, on your next visit you will resume exactly where you left off, with time, mistakes and notes preserved.