Gattai Sudoku Explained: Overlapping Grids and Samurai Layouts

What is gattai sudoku?

Gattai sudoku, from the Japanese word for combining, is any Sudoku built by overlapping two or more classic 9 by 9 grids so they share one or more 3 by 3 boxes. The best known member is the samurai, where a central grid shares its four corner boxes with four outer grids, making a plus shaped board of five puzzles. Larger gattai designs chain many grids together into sprawling constellations. The logic inside any single grid never gets harder than standard Sudoku, so gattai sudoku is intimidating only because of its size, and the shared boxes are the key to taming that size rather than an obstacle to it.

Why the overlaps help you

• A cell in an overlap belongs to two grids at once, so anything you deduce there counts twice.
• Fill a shared box using one grid, and it instantly seeds the neighboring grid.
• The overlap doubles the constraints on those cells, which usually leaves fewer candidates.
• Progress ripples outward from the shared corners toward the edges of each grid.

A worked example in a samurai

Suppose the central grid pins a 6 in a corner box that it shares with the top left outer grid. That 6 now also lives inside the outer grid, where it may complete a line or eliminate the last candidate in a nearby cell. One placement in the seam just advanced two puzzles at the same time. Repeat that across all four corners of a samurai and the whole board unravels in a satisfying spiral. Experienced solvers deliberately open a gattai puzzle by mining the overlaps, because those cells carry the richest information anywhere on the board, and ignoring them means solving five separate puzzles the slow way instead of one connected one.

An order of operations

1. Identify every shared box before writing a single digit anywhere.
2. Solve whichever single grid is most complete to feed values into its overlaps.
3. Push those overlap values into the neighboring grid and look for new singles.
4. Rotate around the shared regions rather than trying to finish one grid alone.

Return to a single elegant grid

Gattai sudoku is best treated as several normal puzzles that talk to each other through the seams, so neat candidate notes and a steady rotation between grids keep the workload manageable even when the board looks enormous. After a five grid samurai, though, the quiet of one board is genuinely welcome. Shapedoku offers exactly that, keeping the classic rules while replacing numbers with nine glowing shapes you recognize by shape and color. Its Notes clean themselves up as you place values, which is a relief after tracking overlaps by hand, and the first hint is always free. Learn to love the seams in gattai sudoku, then enjoy pure, uncluttered deduction at app.shapedoku.com, or check a move with the free solver on shapedoku.com.