How to play Sudoku

Sudoku is the classic number puzzle: a 9x9 grid split into nine 3x3 boxes, with some cells already filled in. Your job is to fill every empty cell so that each row, each column and each box contains the digits 1 to 9 exactly once. There is no arithmetic involved - the digits could just as well be letters or colours. It is pure logic.

Every puzzle here is made fresh by the game when you start it, has exactly one solution, and can be solved by reasoning alone. You never need to guess. The four levels differ in which solving techniques a puzzle demands, from simple scanning on Easy to chains and wings on Crazy.

How to play on skill.games

When you open the game, the settings popup appears. Choose a level under DIFFICULTY, choose how mistakes are treated under MISTAKES, and press PLAY. A new puzzle is generated in a moment.

• The board takes up most of the screen. The digits printed in dark ink are the givens; they can't be changed. The digits you enter appear in blue.
• Choosing a cell: tap any cell. Its row, column and box are shaded, and every cell holding the same digit is highlighted, so you can see at a glance where a digit already sits.
• Entering a digit: with a cell chosen, tap a number on the number pad (under the board, or beside it when your screen is wider than it is tall). The small number under each key shows how many of that digit are still to place; a key fades out once all nine are on the board.
• NOTES: tap NOTES to switch notes on. Numbers you tap are now pencilled into the cell in small print as candidates, and tapping one again removes it. Tap NOTES again to go back to entering digits. When you enter a digit, it is removed automatically from the notes of every cell in its row, column and box.
• ERASE clears the chosen cell (its digit or its notes).
• UNDO takes back your last action - a digit, a note or an erase - all the way back to the start. Undo is free and unlimited.
• Highlighting a digit: with no cell chosen, tap a number on the pad to highlight every cell and note holding that digit. Tap it again to clear the highlight.

On a computer you can also use the keyboard: the arrow keys (or W, A, S, D) move between cells, 1 to 9 enter a digit, Shift and a digit pencil a note, Backspace, Delete or 0 erase, N switches notes on and off, and Z undoes.

The top line shows the clock, the level and your mistakes. The clock runs only while you play: it stops when the menu or a popup is open. The menu button in the top-left corner pauses the game, starts a new puzzle or switches the sound on or off. The game is saved after every move, so if you close the page you can press CONTINUE later and carry on with the same grid, notes, clock and undo history.

When the last cell is filled correctly, the board ripples and the results screen shows your time, your best time at that level (a star marks a new record), your mistakes and the puzzle's code, together with your wins, losses, win rate and streaks.

Rules

The grid. The board has 9 rows, 9 columns and 9 boxes of 3x3 cells. Every row, every column and every box must end up holding each digit from 1 to 9 exactly once.

Givens. The puzzle starts with some digits already placed. They are part of the puzzle and can't be changed. Every puzzle has exactly one way to complete it.

Mistakes - Unlimited (the default). If you enter a digit that is already in the same row, column or box, the clash is marked in red and the mistakes counter goes up by one. Nothing else is checked: a digit that doesn't clash with anything is not marked, even if it turns out to be wrong later - just as on paper. The puzzle is solved when every cell is filled and nothing clashes. Because the solution is unique, a full grid without clashes is always the right one.

Mistakes - 3 lives. Every digit you enter is checked against the solution at once. A wrong digit is marked in red and costs a life; the third mistake ends the game and counts as a loss. Undo can take a wrong digit back, but not the life it cost.

Abandoned puzzles. Starting a new puzzle before finishing the current one counts as a loss in your statistics for that level.

Solving techniques

These are the methods the game's own solver uses to grade every puzzle, from the simplest up.

• Hidden single: a digit that can only go in one cell of a row, column or box. Scan each box for each digit - this alone solves an Easy puzzle.
• Naked single: a cell where only one digit is still possible, because all the others are already in its row, column or box.
• Pointing pairs and box-line reduction: if a digit's only places in a box lie on one row, it can't go anywhere else on that row - and the other way round.
• Naked and hidden pairs: two cells in a unit that can only hold the same two digits claim those digits; two digits that can only go in the same two cells claim those cells.
• Triples and the X-Wing: the same idea with three cells or digits; and a digit confined to the same two columns in two rows can be removed from the rest of those columns.
• XY-Wing: three two-candidate cells linked so that one digit must sit in one of the two outer cells - any cell seeing both can't hold it.
• Advanced: XYZ-Wing, Swordfish, quads, simple colouring and XY-Chains - longer chains of "if this, then that" reasoning, all still without guessing.

Strategy tips

• Scan before you write. Go through the digits 1 to 9 and check each box for a hidden single. It's the fastest way into any puzzle.
• Start where the board is fullest. Rows, columns and boxes with many givens leave few options.
• Use notes once singles run out. Pencil in the candidates of a box or row, then look for pairs and pointing digits among them.
• Keep notes tidy. Entering a digit clears it from its neighbours' notes automatically; erase any note you have ruled out by other means.
• Watch the number pad. A key that shows 1 means a single copy of that digit is still missing - often easy to find.
• Don't guess. Every puzzle here can be solved by logic. If you feel stuck, there is a pattern you haven't spotted yet.

Difficulty levels

The game generates every puzzle on the spot and grades it with a solver that works the way a person does, always using the easiest step available. A puzzle is given to you only if its hardest required step matches the level you chose.

• Easy: at least half the board is filled in (41 of 81 cells); scanning for hidden and naked singles is all you need.
• Normal: at least 40% of the board is filled in (32 of 81 cells); at least one step needs a pointing pair, box-line reduction or a naked or hidden pair.
• Hard: at least 30% of the board is filled in (24 of 81 cells); at least one step needs a triple, an X-Wing or an XY-Wing.
• Crazy: at least 20% of the board is filled in (16 of 81 cells); at least one step needs simple colouring, an XY-Chain, an XYZ-Wing, a Swordfish or a quad.

Statistics and best times are kept separately for each level. If you are new to Sudoku, start on Easy and move up once singles come naturally.