How to play Jigsaw Sudoku

Jigsaw Sudoku, sometimes called irregular Sudoku, keeps the rows and columns of classic Sudoku but throws away the 3x3 boxes. In their place the grid is cut into nine irregular regions of nine cells each, shaped like pieces of a jigsaw. Every row, every column and every region must hold the digits 1 to 9 exactly once. The familiar box patterns no longer work, so you have to read each region's shape and see which rows and columns it reaches.

Every puzzle here is made fresh by the game when you start it - the regions too, so no two boards look alike. Each 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, with a new set of regions, is generated in a moment.

• The board takes up most of the screen. Thick dark lines mark the borders between regions, and neighbouring regions are shaded in different light greys, so you can tell the shapes apart at a glance, even on a small phone. 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 region are shaded, and every cell holding the same digit is highlighted.
• 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 region.
• 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, regions, 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 regions. Each region is a connected group of exactly nine cells - its cells touch side by side, never only at a corner. Every row, every column and every region must end up holding each digit from 1 to 9 exactly once. There are no 3x3 boxes.

The regions. They are drawn at random for every puzzle, and each layout looks the same after a half turn of the board, like the jigsaws printed in puzzle books.

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 region, 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. They all work on regions just as they work on rows and columns.

• Hidden single: a digit that can only go in one cell of a row, column or region. Scan each region for each digit, following its shape rather than a 3x3 block.
• Naked single: a cell where only one digit is still possible, because all the others are already in its row, column or region.
• Region and line intersections: if a digit's only places in a region lie on one row or column, it can't go anywhere else on that line. And if a row's or column's only places for a digit lie inside one region, it can't go anywhere else in that region. Long, thin regions make these crossings very common.
• 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

• Read the shapes first. Before placing anything, look at which rows and columns each region touches. A region that spans only two or three rows or columns forces a lot.
• Use the leftovers. Take a band of three whole rows and the three regions that lie mostly inside it. The band and those three regions both hold every digit exactly three times, so the band's cells outside the three regions must hold exactly the same digits as the regions' cells that stick out of the band. Experienced players use this to pin digits down - the game never requires it, but it can show a step sooner. It works for columns too, and for bands of any width with as many regions as lines.
• Look where regions poke into a line. If a region has only one or two cells in a row, the digits they can take are often limited by the rest of that row.
• Use notes once singles run out. Pencil in the candidates of a region, then look for pairs and digits locked into one row or column.
• 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, follow a region's outline again - its shape usually holds the next step.

Difficulty levels

The game generates every puzzle on the spot, regions included, 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 region-line intersection 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 Jigsaw Sudoku, start on Easy and get used to reading the regions before moving up.