An example Butterfly Sudoku (box-level schematic): each square is one 3×3 box of the 12×12 board. Light-blue corners belong to a single grid, violet edges to two grids, and the four dark central boxes to all four grids at once.
Butterfly Sudoku - one of the Gattai-4 ("four joined") multi-grid puzzles - packs four ordinary 9×9 grids into a single 12×12 square. Each grid slides over its neighbours so that boxes are shared: the two grids on the left and the two on the right meet in the middle three columns, and the top and bottom pairs meet in the middle three rows. A digit in a shared box has to obey every grid that box belongs to.
The layout: four grids in a 12×12 square
Number the 12×12 board as a 4×4 arrangement of 3×3 boxes. The four 9×9 grids are the four overlapping 3×3-box blocks: top-left covers box-columns 1-3, top-right covers box-columns 2-4, and the two lower grids sit three rows down. Because each grid is nine cells wide inside a twelve-wide board, adjacent grids share their middle two box-columns (or box-rows), and the four boxes in the very centre fall inside all four grids at once.
A worked example, step by step
Step 1 - start in the shared boxes. A digit placed in one of the four central boxes counts as a given for every grid that box belongs to. Those four boxes, shared four ways, are the richest cells on the board, so mine them first.
Step 2 - solve each grid as a normal Sudoku, but glance across. Treat the four grids as four ordinary 9×9 puzzles. Each time a shared box advances, carry the new digits straight into the neighbouring grid that also owns that box.
Step 3 - let the corners feed the centre. The lightly-shared corner boxes are the only cells private to one grid; finish enough of a grid's private half and it fixes digits in the overlap, which then unlocks its neighbours.
Solving strategy
- Anchor on the centre. The four central boxes are shared four ways, so every digit there is the most constrained - and the most productive - placement on the board.
- Finish one grid to seed the next. Completing any grid hands its shared boxes to two neighbours, which cascades across the butterfly.
- Keep the four grids straight. A row belongs to exactly one grid; the only thing shared between grids is the boxes, never whole rows or columns.
Common mistakes
- Reading a row across the whole 12×12. Rows and columns stop at each grid's own nine cells - only the boxes overlap, so a "1-9" line never spans the full width.
- Solving a shared box for one grid only. A central box must satisfy all four grids it sits in; a placement legal in one but illegal in another is wrong.
- Ignoring a corner grid. Neglect one grid and you starve the overlap it feeds, stalling the neighbours that share its boxes.
Can I play it here?
Not yet - Butterfly Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Try the closely related Samurai, which you can play right now.
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Butterfly Sudoku belongs to the overlapping family and runs on 12×12 (four overlapping 9×9). It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Easy Sudoku leaves 36 of the 81 cells filled, so there is always an obvious next move somewhere; Hard Sudoku leaves 26, where scanning gets you started and then leaves you stranded. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.