X Sudoku 9×9 Extreme — Set 21
Solver Notes
Extreme 9x9 X-Sudoku — the hardest diagonal sudoku available. Forcing chains and multi-step coloring required. 50+ minutes for experienced solvers.
X-Sudoku — Free Online & Printable
X-Sudoku adds a diagonal constraint to the classic sudoku rules. Both main diagonals — running from top-left to bottom-right and from top-right to bottom-left — must each contain the numbers 1 through 9 without repetition. This is the most accessible sudoku variant: the two extra constraints actually make many cells easier to deduce, since numbers on the diagonals are additionally restricted. If you''ve mastered standard sudoku and want a fresh twist without learning complex new rules, X-Sudoku (also called Diagonal Sudoku) is the ideal starting point.
About 9x9 Grid
The 9x9 grid is the full standard size for all sudoku variants. With 81 cells in 3x3 boxes, a 9x9 variant puzzle takes 10 to 60+ minutes depending on difficulty level and variant type. All variant rules reach their full expressive potential at this size: Killer cages create complex arithmetic networks, Jigsaw regions form visually distinctive shapes, and diagonal constraints span the entire grid. The 9x9 is where experienced solvers spend most of their time, and where the depth of each variant is fully revealed.
Extreme Difficulty
Extreme difficulty represents the hardest puzzles solvable through pure logic — no guessing required, but the reasoning chains are exceptionally long. Required techniques include forcing chains (sequences of implications that eliminate candidates by showing they lead to contradictions), multi-step coloring, and in some cases trial-and-error with backtracking when chains become impractical to track mentally. These puzzles are designed for experienced competitors and dedicated enthusiasts. Average time: 60+ minutes. Even experienced solvers may spend 10-15 minutes without making a single cell entry while planning a deduction chain.
How to Get Started
- Choose "New Puzzle" to generate a fresh puzzle with a unique solution
- Click or tap cells to enter numbers (1-9 for 9x9)
- Use the constraint highlights to track which numbers are still available
- Print the puzzle if you prefer solving on paper