Choose a level from the tabs above the board: there are 10 puzzles at each of easy, medium and hard. Expect a samurai to take several times as long as a single grid, and to reward patience more than speed.
The rules
- Each of the five 9×9 grids follows classic sudoku rules: the digits 1 to 9 once in every row, every column and every 3×3 box of that grid.
- The four corner boxes of the center grid are shared. A digit placed there counts for the center grid and for the corner grid it overlaps.
- Rows and columns belong to one grid only. A corner grid's row ends at the edge of that grid, even where it lines up with a row of the center grid.
- Some digits are given at the start. Every puzzle has exactly one solution.
Where to start
Begin in whichever grid has the most givens, and solve it as far as it will go. Then look at the shared boxes. A cell in a shared box sees two sets of rows and columns, one from each grid, so it often runs out of candidates earlier than anything else on the board.
When a grid stalls, move on rather than staring at it. Progress in a corner grid fills its shared box, and those digits are new information for the center. The center then feeds the other three corners. Most samurai puzzles are solved in loops like this, going round the board two or three times.
Pointing works across grids too. If, inside a shared box, every possible place for a 4 lies in one row of the board, then 4 is ruled out of that row in both grids that run through the box.
How easy, medium and hard differ
Easy samurai puzzles need only naked and hidden singles. The challenge is purely the size: lots of digits to find, but each one can be found by scanning.
Medium puzzles add pointing and claiming and naked pairs. You'll need pencil marks, at least in the shared boxes, to see them. Hard puzzles bring hidden pairs, X-wings and XY-wings. On a board this large those patterns can sit in any of five grids, so a methodical sweep through each grid in turn finds them faster than hunting at random.
Keeping track on a big board
With 369 cells, bookkeeping matters more than cleverness. Pencil in candidates for the shared boxes first and leave the outer corners of each grid until they are needed. When you finish a grid, say so to yourself and stop checking it; the remaining four are where the work is.
If you play on screen, the Pencil button keeps candidates separate from placed digits, and Undo lets you back out of a wrong turn without losing your place. On paper, a sharp pencil and a light hand do the same job.
Learning and printing
The how to play samurai sudoku guide explains the overlap in more detail, and the sudoku variants overview compares samurai with killer and jigsaw sudoku if you want to try something else.
A samurai board is large, so it is a puzzle many people prefer on paper. The printable samurai sudoku packs are PDFs with the answers on the last pages. If you enjoy irregular shapes as well as scale, try jigsaw sudoku next.