Samurai Sudoku: Rules, Layout and How to Solve It

Samurai Sudoku looks scary the first time you see it: five Sudoku grids tangled together into one big cross. But there is no new rule to learn. If you can solve a classic 9×9, you can solve a Samurai. It just asks you to keep two grids in mind at once and to stay patient. Here is how it works, where the name comes from, and a calm strategy to finish one.

What is Samurai Sudoku?

How the five grids fit together

Picture a square 21 cells wide and 21 cells tall. Four 9×9 grids sit in its corners, and a fifth grid sits right in the centre. The centre grid’s four corner boxes are the same cells as the inner corner box of each outer grid. That arrangement of four corners plus a centre is called a quincunx.

Where does 369 come from? Five grids of 81 cells make 405, but each of the four shared boxes would be counted twice, so you take away 4 × 9 = 36. That leaves 405 − 36 = 369 cells to fill, about four and a half classic puzzles’ worth.

Notice the empty spaces too. Between the two top grids, rows 1–6 of columns 10–12 belong to no grid at all. The same is true on the other three sides. Those gaps are blank paper, not cells.

The rules of Samurai Sudoku

The rules are the classic ones, applied to each grid separately:

1. Each of the five grids is a normal 9×9 Sudoku with nine rows, nine columns and nine 3×3 boxes.
2. In every grid, each row, column and box must contain the digits 1 to 9 exactly once.
3. A cell in a shared corner box belongs to two grids, so its digit must work in both of them.
4. Rows and columns stop at the edge of their own grid. They don’t continue across the gaps into the next grid.

A good Samurai, like a good classic puzzle, has exactly one solution. The given digits are spread across all five grids, and often the shared boxes start with few or none, so you have to earn them.

Where the name comes from

In Japan, overlapping Sudoku puzzles are known as gattai, a word meaning “merged” or “combined”. The number after it says how many grids are joined, so the five-grid cross is Gattai 5. English-language newspapers such as The Times, The Age and The Sydney Morning Herald have printed it under the name Samurai Sudoku, which is the name most players know today. Some papers use other names: the Baltimore Sun and the Toronto Star run it as “High Five” on Sundays.

Who designed the first one, and exactly when, is not clearly recorded, so we won’t pretend to know. What is clear is that it arrived in the West alongside the classic Sudoku boom of the mid-2000s. For the bigger story of how Sudoku spread from puzzle magazines to newspapers around the world, read our history of Sudoku.

A step-by-step solving strategy

There is no special Samurai technique. You use the same moves as in a classic puzzle; the art is in choosing where to look. This order works well:

1. Survey all five grids first

Before writing anything, glance over each grid and count its givens. Some grids start rich and some start thin. Begin with the grid that has the most digits, because it will give you the quickest progress and feed its neighbours.

2. Hunt for easy singles in each grid

Inside each grid, scan box by box for hidden singles, exactly as you would in a classic puzzle. This is where most of your early progress comes from.

In a Samurai you would run this same scan in every box of every grid. The move doesn’t change; there are just more boxes to visit. Our Sudoku strategies guide walks through this and nine more techniques on real puzzles.

3. Attack the shared corner boxes

The four shared boxes are the most powerful cells in the puzzle. Each one sits inside two grids, so it is squeezed by two sets of rows and two sets of columns. A digit can be ruled out of a shared cell by something in the outer grid or by something in the centre grid. That double pressure means shared boxes often solve sooner than you’d expect once each neighbouring grid has a few digits in place.

4. Carry digits across the overlap

Every time you fill a shared cell, stop and ask: “What does this change in the other grid?” A digit placed in the top-left grid’s corner box is also a digit in the centre grid’s corner box, so it now blocks that number from a centre row and a centre column. That can open a single somewhere in the centre, which may in turn fill another shared box and pass the favour to a different outer grid. Solving a Samurai is mostly about letting this chain reaction run.

Pencil marks help here. If you can show in the centre grid that a 5 in its top-left box must sit in row 7 (a pattern called locked candidates), then the top-left grid’s row 7 can’t have a 5 anywhere outside that box either. The logic is the same as in a single grid; it just reaches further.

5. Work the centre grid

The centre grid touches all four others, so it is the hub. Once two or three outer grids have fed it some digits, give it your full attention. Progress in the centre flows outward through all four corners at once, and that is often what breaks a stuck outer grid open.

6. Finish one grid at a time

When a grid is close to done, finish it before moving on. A complete grid is one less thing to keep in your head, and its shared box is now fixed for its neighbour. Then repeat: pick the next richest grid, scan, carry digits across, and keep going. If you get stuck everywhere, go back to the shared boxes and write full pencil marks there.

Common mistakes to avoid

• Checking only one grid. Before you write a digit in a shared box, check its row, column and box in both grids it belongs to. It’s the most common error, and a wrong digit in a shared box spoils two grids at once.
• Reading rows across the gap. Row 1 of the top-left grid and row 1 of the top-right grid line up on the page, but they are different rows. A 7 in one says nothing about the other.
• Treating each grid as a separate puzzle. An outer grid on its own may not have enough clues to finish. Expect to need help from the centre, and give help back.
• Forgetting to update pencil marks. When you place a digit in a shared box, clear that digit from the notes in both grids, not just the one you were working in.
• Guessing when stuck. With 369 cells, an early guess can take a long time to show itself as wrong. A well-made Samurai can be solved by logic, so look again at the shared boxes instead.
• Trying to finish in one sitting. A Samurai is a marathon. It’s fine to put it down and come back with fresh eyes, which often spot the move you missed.

Other overlapping Sudoku puzzles

Samurai is the best-known member of the gattai family, but grids can be merged in many other shapes. A few you may meet in puzzle books and apps:

The tighter the overlap, the more each cell is squeezed by several grids. Butterfly and Flower puzzles feel denser and more connected than a Samurai, while a Twodoku is a gentle way to try overlapping grids for the first time. For a tour of variants that change the rules rather than the shape, such as Sudoku X, Jigsaw and Thermo, see our guide to Sudoku variants. If one extra rule appeals more than five grids, start with diagonal Sudoku (Sudoku X). If sums appeal to you, try Killer Sudoku next.

How to get ready for Samurai Sudoku

A quick, honest note: Sudoku.my currently offers classic 9×9 puzzles, not Samurai. But classic puzzles are the best training there is, because a Samurai is made of nothing else. Every skill you need, such as hidden singles, pencil marks, locked candidates and pairs, you can practise one grid at a time.

• Build stamina with hard puzzles. On Sudoku.my, hard puzzles call for pairs and triples, the kind of steps that unlock a stubborn Samurai grid.
• Get comfortable with notes. Shared boxes are much easier with good pencil marks. Our guide to Sudoku pencil marks shows a tidy system, and the app’s notes remove candidates for you as you place digits.
• Play the daily challenge. It starts gentle on Monday and grows tougher towards the weekend, a good weekly rhythm for building up to longer puzzles. Every puzzle has exactly one solution and can be solved by logic, just like a good Samurai.
• Learn from the hints. Each hint explains the move it suggests, so when you’re stuck you learn the pattern rather than just getting a digit.

Miko the cat recommends one more habit: when a classic puzzle feels easy, try solving it without notes for the first few minutes. Holding facts in your head is exactly what carrying digits across a Samurai’s overlaps asks of you.

#Samurai Sudoku#Variants#Gattai