All Sudoku Variants
12×12 (four overlapping 9×9) · Overlapping

Butterfly Sudoku

Four 9×9 grids packed into a 12×12 square so each overlaps its neighbours — the four central boxes belong to all four grids at once, and every grid is a full Sudoku.

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: