Samurai Sudoku

Samurai Sudoku: five grids, four shared boxes

A centre grid with four more overlapping it at the corners. Each of the four outer grids shares one entire 3×3 box with the middle one, which is what turns five separate puzzles into a single object.

Those four shared boxes are the whole game. A digit placed in the top-left grid's bottom-right box is simultaneously a digit in the centre grid's top-left box, and it has to satisfy both grids at once. Work the outer grids in isolation and you will stall; work the overlaps and the thing unlocks.

How the puzzles are made

Each of the five grids is generated as a complete sudoku and then has its clues carved away one at a time, with the puzzle re-solved after every removal and the clue put back whenever a second solution appears.

Because each sub-grid is independently unique, the whole board is too: the shared boxes cannot introduce ambiguity, they can only remove it. So there is exactly one correct completed board, and the printed solution is it.

Choices

One or two boards per download, and the solutions toggle. There is no difficulty control, and that is not an omission — a Samurai board is long by construction, and the difficulty a solver actually experiences comes from the overlaps rather than from the clue count in any one grid.

Each board fills its own page. At two per download you get two pages of puzzle, plus two more if solutions are on.

Expect it to take a while

Five grids means 369 cells, not 405, because the four shared boxes are counted once rather than twice. A confident sudoku solver should budget an hour and will probably want two sittings.

That length is the appeal. It is a puzzle for a long train journey, a wet Sunday, or the fortnight after Christmas, and it does not really work as a ten-minute filler.

How to approach it

Start in the centre grid. It touches all four others, so every digit you place there does double duty.

Then work outward through the shared boxes rather than finishing one grid at a time. The temptation is to complete the top-left grid because it looks like a normal sudoku sitting on its own; resist it, because the information that would have made it easy is sitting in the centre grid you have not touched.

Keep a pencil and keep candidate notes small. On a board this size, rubbing out is the main cost.

Print at 100% on A4 or US Letter. The grids are drawn as large as a single sheet allows, which is not very large — a 0.5mm pencil is a genuine help here.