X Sudoku 9×9 Hard — Set 31
Solver Notes
Hard 9x9 X-Sudoku requires tracking constraint chains across diagonals and boxes. Naked pairs and pointing pairs become essential. 20-35 minutes.
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.
Hard Difficulty
Hard difficulty introduces multi-step deductions where you must track constraint chains across multiple regions simultaneously. Required techniques include naked pairs/triples (two or three cells sharing the same possible values), pointing pairs (where a candidate is confined to one row or column within a region), and intersection removal. For variant-specific techniques: Killer Hard requires cage combination analysis where several possibilities exist and must be eliminated systematically. Average time: 20-40 minutes for 9x9.
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