All Sudoku Variants
Four overlapping 9×9 (ring) · Overlapping

Sohei Sudoku

Four 9×9 grids set as a ring — top, bottom, left and right — each sharing one corner box with each neighbour, around a hollow centre that belongs to no grid.

An example Sohei Sudoku (box-level schematic): each square is one 3×3 box. The four grids (blue) form a cross; the four dark boxes are each shared by two grids, and the grey centre box belongs to no grid at all.

Sohei Sudoku — named for the warrior monks of medieval Japan — is a Gattai-4 ("four joined") puzzle that arranges four ordinary 9×9 grids in a ring. One grid sits at the top, one at the bottom, one left and one right, joined corner to corner like linked hands. At the very middle there is a hollow: a 3×3 area that belongs to no grid and is never filled.

The layout: four grids around a hollow centre

Each grid overlaps its two neighbours by exactly one 3×3 box, and only at the corners. The top grid's bottom-left box is shared with the left grid; its bottom-right box is shared with the right grid. The bottom grid shares its top-left and top-right boxes with the left and right grids the same way. Those four shared boxes ring the empty centre, so — unlike Samurai — no grid touches the middle, and a row or column can never run straight through from one side to the other.

A worked example, step by step

Step 1 — work the four shared corners first. Each dark box sits in two grids at once, so any digit there is instantly a given for both. These four boxes are the only bridges between grids, and they are where cross-grid deductions begin.

Step 2 — solve each grid as a normal Sudoku. Treat the four grids as four ordinary 9×9 puzzles. Whenever a shared box advances, carry the new digits into the neighbouring grid that also owns it.

Step 3 — go around the ring. Progress travels the loop: the top grid feeds the left, the left feeds the bottom, the bottom feeds the right, and the right feeds the top again. Keep circling until every grid is full.

Solving strategy

  • Live in the four overlaps. They are the only shared cells, so every digit placed there counts twice and is the highest-value move on the board.
  • Never cross the hollow. The empty centre breaks every line of sight — opposite grids are linked only the long way around the ring, never straight through the middle.
  • Follow the loop. A stalled grid usually restarts once its neighbour on the ring hands it a fresh digit through their shared box.

Common mistakes

  • Filling the centre. The grey box is not a puzzle region — it holds no digits and imposes no constraint.
  • Reading a line across two grids. Only the corner boxes are shared; rows and columns stop at each grid's own nine cells and never continue through the hole.
  • Solving a shared box for one grid only. A corner box must satisfy both grids it belongs to — a placement legal in one but illegal in the other is wrong.

Can I play it here?

Not yet — Sohei 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: