Diagonal Sudoku: Rules & Strategies

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What Is Diagonal Sudoku?

The Standard Sudoku Rules Still Apply

Every row, column, and 3×3 box must contain the digits 1–9, each exactly once. This never changes. If you know standard Sudoku rules, you already know the foundation.

The One Extra Rule: Both Main Diagonals

In Diagonal Sudoku, the two main diagonals of the 9×9 grid are treated as additional constraint groups, just like rows and columns. Specifically:

• Top-left to bottom-right diagonal (cells R1C1 → R2C2 → R3C3 → … → R9C9)
• Top-right to bottom-left diagonal (cells R1C9 → R2C8 → R3C7 → … → R9C1)

Each diagonal must contain the digits 1–9 with no repeats. That's the only difference from standard Sudoku.

The name "X-Sudoku" comes from the visual: if you highlight both diagonals on the grid, they form the shape of an X.

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Which Cells Are on the Main Diagonals?

The two diagonals share exactly one cell: the centre of the grid at R5C5. That cell sits in four constraint groups simultaneously — its row, its column, its 3×3 box, and both diagonals. It's often one of the first cells to resolve in a diagonal puzzle.

Nine cells form the top-left diagonal: R1C1, R2C2, R3C3, R4C4, R5C5, R6C6, R7C7, R8C8, R9C9.
Nine cells form the top-right diagonal: R1C9, R2C8, R3C7, R4C6, R5C5, R6C4, R7C3, R8C2, R9C1.

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Why the Diagonal Constraint Is So Powerful

You Get 18 Extra Placements for Free

Standard Sudoku has 27 constraint groups (9 rows + 9 columns + 9 boxes). Diagonal Sudoku has 29 (the same 27 plus 2 diagonals). More constraints = more logical deductions = faster progress. Puzzles feel different from the start: the diagonal cells resolve earlier, which cascades into the rest of the grid.

Diagonal Cells Sit in Three Constraint Groups Simultaneously

A cell on one of the main diagonals belongs to its row, its column, its box, and its diagonal. Corner cells like R1C1 sit in only the one diagonal, but centre-adjacent cells like R4C4 overlap with box 5 and the main diagonal, creating dense constraint clusters. This is where most early breakthroughs happen.

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Four Strategies for Diagonal Sudoku

1. Start with Diagonal–Box Intersections

The corner boxes (top-left, top-right, bottom-left, bottom-right) each share three cells with a diagonal. Use those shared cells to cross-eliminate: if a digit appears in a corner box, it's eliminated from the rest of that diagonal.

2. Use the Diagonal as a Virtual Row or Column

Once you treat the diagonal as just another "row," all standard techniques apply — naked singles, hidden singles, naked pairs, pointing pairs — but across the diagonal cells instead of a horizontal or vertical line. Scan the diagonal the same way you'd scan row 5.

3. Cross-Reference Both Diagonals

The diagonals share R5C5. If you've placed a digit on one diagonal, that digit is eliminated from the other diagonal's pool for R5C5. Even partial placement on one diagonal gives you information about the other.

4. Apply Standard Techniques First

Don't over-rotate into diagonal thinking before exhausting standard logic. The diagonals provide bonus constraints, but rows, columns, and boxes often solve first. Treat diagonal deductions as a secondary scan — run them after you've applied naked singles, hidden singles, and naked pairs to the whole grid.

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Explore More Sudoku Variants

• Killer Sudoku — Rules, combinations table, and strategies for the world's most popular variant.
• Thermo Sudoku — Thermometer ordering constraints and the strategies that crack them.
• Arrow Sudoku — How arrow sum constraints work and how to exploit them.
• All Sudoku Variants — Every variant guide in one place.
• Sudoku Strategies — Row, column, and box techniques that apply to every variant.

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Frequently Asked Questions

Is Diagonal Sudoku harder than regular Sudoku?

Not necessarily — it depends on the puzzle. The extra diagonal constraints add information, which can actually make some steps easier than in standard Sudoku. That said, puzzle constructors often exploit the diagonals to create hard eliminations that don't exist in standard grids. Expect a different kind of challenge rather than a uniformly harder one.

Is X-Sudoku the same as Diagonal Sudoku?

Yes. X-Sudoku and Diagonal Sudoku are the same puzzle. The "X" refers to the visual shape formed when both main diagonals are highlighted on the grid. Some puzzle books and apps use one name; others use the other. You may also see it called "Diagonal X-Sudoku."

Does the R5C5 centre cell always resolve early?

Often, yes. The centre cell sits at the intersection of both diagonals, its row, its column, and the centre box — five constraint groups. That density makes it one of the most restricted cells in the grid, and it frequently becomes a naked single within the first few solving steps.

Can I use standard Sudoku strategies on Diagonal Sudoku?

Absolutely. Every standard technique — naked singles, hidden singles, naked pairs, pointing pairs, X-wings — works exactly the same way, with the diagonals added as two extra groups. The strategy set doesn't change; you just have two more lines to scan.

Where can I play Diagonal Sudoku puzzles?

Diagonal Sudoku puzzles appear on several puzzle sites and in dedicated puzzle apps. For a clean daily Sudoku habit with no ads, try Sudoku a Day — it's a good foundation for building the elimination skills that transfer directly to diagonal variants.