Sudoku X Sudoku: The Diagonal Sudoku Variant That Adds an Extra Twist
X Sudoku — also known as Diagonal Sudoku — is one of the most popular and elegant Sudoku variants. It takes the classic 9×9 grid you already know and adds a single, powerful new rule: both main diagonals must also contain the digits 1 through 9 exactly once. The diagonals form an "X" across the grid, giving this variant its name. It's a fresh challenge that rewards careful thinking and opens up entirely new solving strategies.
🤔 What Is X Sudoku?
X Sudoku uses the same 9×9 grid divided into nine 3×3 boxes as regular Sudoku. The grid also starts with pre-filled given digits. What makes it special is the diagonal constraint: the two diagonals running from corner to corner act as two extra "regions" — just like rows, columns, and boxes. On our game board, diagonal cells are highlighted with a subtle shaded background so you can always see which cells carry the extra constraint.
X Sudoku is sometimes called "Sudoku X", "Diagonal Sudoku", or "Argyle Sudoku" when extended to all broken diagonals. The simple two-diagonal version is the most widely published and played. It first gained popularity in European puzzle magazines in the early 2000s.
📋 Rules of X Sudoku
X Sudoku follows four simple rule sets:
- Row rule — Every row must contain the digits 1–9 exactly once.
- Column rule — Every column must contain the digits 1–9 exactly once.
- Box rule — Every 3×3 box must contain the digits 1–9 exactly once.
- Diagonal rule — The main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each contain the digits 1–9 exactly once.
Every puzzle has exactly one valid solution reachable through pure logic — no guessing needed.
The centre cell (row 5, column 5) sits on both diagonals, its row, its column, and its 3×3 box — making it the most constrained cell on the board. Figuring out this cell early often unlocks a cascade of deductions.
⭐ Difficulty Levels Explained
Our X Sudoku offers four difficulty levels:
- Easy — Plenty of given digits (around 38). The diagonal constraint helps rather than hinders, making cells easier to deduce. Perfect for your first diagonal puzzle.
- Medium — Fewer givens (around 30). You'll need to actively use diagonal elimination alongside standard techniques. A great daily challenge.
- Hard — Around 25 givens. Requires combining diagonal logic with advanced strategies like naked pairs and pointing pairs.
- Expert — Minimal givens (around 21). Demands deep mastery of both standard and diagonal-specific techniques.
🧠 Essential X Sudoku Strategies
All standard Sudoku strategies apply, but the diagonals give you extra tools:
1. Diagonal Scanning
Treat each diagonal like an extra row. Scan both diagonals to see which digits are placed and which are missing. This often reveals cells that can't be solved with rows, columns, and boxes alone.
2. The Centre Cell Trick
The cell at position (5, 5) belongs to five regions: its row, its column, its 3×3 box, the main diagonal, and the anti-diagonal. With so many constraints, this cell is frequently determinable early in the solve.
When you place a digit on a diagonal, immediately eliminate that digit from all other cells on the same diagonal. This simple step can cascade into multiple placements — diagonal elimination is surprisingly powerful.
3. Diagonal-Box Interaction
Where a diagonal passes through a 3×3 box, it creates a strong interaction. If a digit is missing from both the diagonal and the box, and it can only go on diagonal cells within that box, you can eliminate it from the rest of the diagonal.
4. Corner & Edge Logic
The four corner cells of the grid each sit on a diagonal plus their row, column, and box. These highly constrained cells are often solvable before interior cells.
5. Naked Pairs on Diagonals
The standard naked pairs and triples techniques extend to diagonals. If two cells on the same diagonal share the same two-only candidates, those digits can be eliminated from all other diagonal cells.
Because the diagonal constraint is so powerful, X Sudoku puzzles can have fewer given digits than classic Sudoku and still have a unique solution. Some valid X Sudoku puzzles have as few as 14 givens — compared to the minimum 17 for standard Sudoku!
🆚 X Sudoku vs. Regular Sudoku
How do the two compare?
- Extra constraint: X Sudoku adds two diagonal regions. Regular Sudoku has 27 constraint groups (9 rows + 9 columns + 9 boxes); X Sudoku has 29.
- Solving feel: The diagonals provide extra information, which can make some cells easier to solve — but you must remember to check diagonals alongside rows, columns, and boxes.
- Number of valid grids: There are far fewer valid X Sudoku grids than standard Sudoku grids, making puzzle construction more challenging.
- Visual appeal: The highlighted diagonal cells give the grid a distinctive X-shaped pattern that's instantly recognisable.
📜 A Brief History of X Sudoku
Diagonal Sudoku first appeared in puzzle magazines in the late 1990s, shortly after standard Sudoku gained popularity in Japan. European puzzle publishers — particularly in Germany and the Netherlands — were among the first to feature it regularly. The name "X Sudoku" became standard because the two diagonals visually form the letter X across the grid.
The variant gained widespread recognition after being featured in the World Puzzle Championship, where it has been a recurring puzzle type since the mid-2000s. Today, X Sudoku is found in newspapers, puzzle apps, and competition booklets worldwide.
If you're transitioning from classic Sudoku, try X Sudoku on Easy first. The diagonals provide helpful extra constraints that actually make the puzzle more approachable — you might be surprised how naturally diagonal scanning fits into your solving workflow.
💪 Benefits of Playing X Sudoku
- Expands your solving toolkit — learning to think diagonally adds a new dimension to your logical reasoning.
- Improves spatial awareness — tracking diagonals alongside rows, columns, and boxes strengthens visual-spatial thinking.
- Fresh challenge for Sudoku veterans — if classic Sudoku feels routine, X Sudoku injects new life without completely changing the rules.
- Great competition prep — diagonal puzzles appear frequently in puzzle championships, so practising here sharpens your competitive edge.
🎮 More Sudoku Variants to Explore
- Classic 9×9 Sudoku — The original puzzle. Master the fundamentals first.
- Killer Sudoku — Cage sums replace given digits for an arithmetic twist.
- Jigsaw Sudoku — Irregular regions replace the standard 3×3 boxes.
- Samurai Sudoku — Five overlapping grids for the ultimate challenge.
Frequently Asked Questions
X Sudoku (Diagonal Sudoku) is a variant where, in addition to the standard row, column, and 3×3 box rules, both main diagonals must also contain the digits 1–9 exactly once. The diagonals form an X across the grid.
Fill every row, column, and 3×3 box with 1–9. Additionally, both main diagonals (top-left to bottom-right and top-right to bottom-left) must each contain 1–9 exactly once.
It's different rather than strictly harder. The diagonals add extra constraints, giving you more information to work with — but you also need to track two additional regions. Most solvers find it a refreshing change of pace.
The two main diagonals run corner-to-corner, forming an X. Each diagonal must contain all digits 1–9 with no repeats — just like a row or column. Diagonal cells are highlighted on the board with a shaded background.
Yes, 100% free with no sign-up, ads-wall, or paywall. Just open the page and start playing immediately.