How to Solve Samurai Sudoku (Five 9x9 Grids) on Paper with a Pen
NetSudoku Editorial Team
Reading time 8 min ยท Technique Guide
Table of Contents โผ
Samurai Sudoku โ the five-grid gattai โ is the marathon of the pen-and-paper world. Five overlapping 9x9 grids share four 3x3 corner boxes to form a single board of 369 cells. Only classic rules apply, but 369 cells across five grids must be tracked, and every digit you place in one grid reaches into its neighbours without asking permission. There is no amicable middle ground and no undo button.
That makes Samurai the ultimate stress test for the discipline built across the whole pen-and-paper Series Hub. The placement-time strikes, the candidate hygiene, the 45-check โ all of it now runs five boards deep, connected by four shared boxes that function as plumbing between grids. Read the hub for the universal method, then come back here to scale it to 369 cells. The classic technique stack in the strategy index and the X-Wing technique guide still applies, now multiplied across five boards.
The no-undo discipline, amplified: a digit placed in the central grid's corner box is a fact in two grids at once. Every commitment you make in Samurai is five times as binding โ so every commitment must be five times as verified.
1. The Precise Rules
- Five 9x9 grids, each subject to classic rules: every row, column, and 3x3 box contains digits 1โ9 exactly once.
- A central grid's four corner 3x3 boxes are shared with the corner boxes of four surrounding grids (left, right, top, bottom).
- Total cells:
5 ร 81 โ 4 ร 9 = 369. - Each shared 3x3 box is one 1-9 set belonging to two grids simultaneously โ a single solution, two sets of rows, columns, and boxes referring to it.
- Only classic rules apply. There is no extra constraint beyond the overlap itself; the challenge is sheer scale and cross-grid propagation.
Every house still sums to 45 โ that fact never changes. In Samurai it becomes your audit backbone: the four shared boxes sum to 45 once but serve two grids, so a digit placed there must satisfy both houses of both grids.
2. Setting Up 369 Cells on Paper
You cannot put five overlapping 9x9 grids cleanly on one A4 sheet โ not legibly, and not with a chair. Before solving, lay out the battlefield:
- Print or tape the gattai. Assemble the five grids onto a single working surface, taping pages together where the overlap demands more width than one sheet gives.
- Label each sub-grid in the margin. Write N, E, S, W, and C (for centre) clearly so any spoken reference โ "the N grid's right box" โ resolves unambiguously.
- Agree on one shared coordinate language. Name each overlap 3x3 box once (say, the NW overlap, NE overlap, SW overlap, SE overlap) so a placement in the central grid maps cleanly to the outer grid's corner box.
3. Step-by-Step Workflow
Pass 1A โ The Four Outer Corners, Inward
Attack the outer corner grids first, driving each one toward its shared box. The shared 3x3 boxes are the plumbing โ fill the feeders before you stress the junction, and the centre falls into place with fewer guesses.
Pass 1B โ The Central Grid
With the four outer grids reduced, move to the centre. Every placement in C propagates through the four shared boxes into the outer grids โ and every outer-grid placement constrains C. Alternate between them as the flows clear.
Pass 2 โ Candidate Markup Across All Live Grids
Mark candidates grid by grid, in the outer grids first. Keep the shared boxes in a single ledger: a candidate struck in the box is struck for both grids. Let each grid's own notation sit within its region.
Pass 3 โ Propagation and Audit
Apply the reductions of the toolkit, then verify every house of every grid sums to 45. The solve-state checklist tells you which grids still carry empty cells; never declare a grid done until its 45-audit passes.
4. The Samurai Toolkit
4.1 Attack Order: Outer Corners Toward the Centre
The four outer grids share exactly one box each with the centre. By solving an outer grid's non-overlapping cells first, you hand the shared box a near-complete set, and the centre inherits a nearly finished boundary. The shared 3x3 boxes are the plumbing between grids โ feed them first and the flow into C does the hard work for you.
4.2 The 45-Rule Propagation
Every house โ every row, column, and box, in every one of the five grids โ sums to 45. That invariant never breaks. So a placement in the central grid propagates through the four shared boxes into the outer grids, and a placement in an outer grid's shared box propagates back into C. Where classic boards use the 45-rule to audit, Samurai uses it to propagate: sum-check a shared box and you are simultaneously auditing two grids.
4.3 The Shared Box as a Double Set
A shared 3x3 box is the same 1-9 set in two grids at once. A digit solved in the central grid instantly solves or constrains the identical cell in the outer grid's corner box, and vice versa. Treat the four overlaps as single ledgers that belong to two grids โ recording a digit there writes the same fact into two houses of rows and columns simultaneously.
4.4 Pacing and the Solve-State Checklist
With 369 cells, momentum is everything. Keep a running checklist โ which of the five grids still live in Pass 2, which shared boxes are unresolved. Grids stall because the solver drifts across the giant board without a plan. The checklist restores order and tells you where to look next.
5. The Five-Grid Layout and One Overlap
The schematic shows the plus arrangement of five grids. The four tinted 3x3 boxes are the overlaps where a single digit serves two grids at once.
Read the overlap this way: the central grid C shares its four corner 3x3 boxes with the outer grids. Place a 7 in the NW corner box of C โ it is simultaneously the NE corner box of W's board. That one digit must satisfy the row and column houses of W and the row and column houses of C, and it must hold in both boxes' 1-9 sets. Because both boxes sum to 45 across the same nine cells, a digit settled there propagates both ways at zero extra cost.
6. Common Samurai Mistakes
- Solving the centre too early. The central grid is held hostage by its four shared corners. Drive the outer grids inward first, then let C finish itself from well-fed boundaries.
- Forgetting the double set. A digit in a shared box is one fact in two grids. Missing the second household is how a perfectly solved C crashes an outer grid.
- Losing the coordinate language. Without agreed overlap names, placements blur across grids. Label N/E/S/W/C and the four overlaps before you place a single digit.
- Neglecting the 45-audit. With 369 cells, a single house break snowballs across five grids. Verify every house sums to 45 before you call any grid done.
7. Train the Marathon
Do not begin Samurai on a full 369-cell board. Rehearse the propagation mechanics on Easy and Medium single grids first, then scale the same workflow outward. When you hold clean candidate notation on a single Hard or Expert board, the five-grid gattai becomes a matter of pacing, not new logic. Revisit the X-Wing technique guide and the strategy index for the reductions that scale across the overlaps.
That closes the pen-and-paper series. From the single 9x9 of Part 1 to the five-grid gattai of Part 16, every variant reduces to the same discipline: mark it, scan it, strike it, and verify the 45. Carry the method and no printed grid โ however many cells โ will ever look untameable again.
Back the Marathon With Validated Boards
Every NetSudoku puzzle โ from classic to the scale of cross-grid propagation โ is algorithmically generated with a single unique solution, so a 369-cell truth always has a verifiable answer key.