X-Sudoku
Classic rules, plus a hidden twist: each of the two main diagonals must also contain every digit 1–9.
What makes it X-Sudoku
The two main diagonals — top-left to bottom-right and top-right to bottom-left — become unique units in their own right. That's 11 constraints per cell on the diagonals, instead of the usual 3. It changes which cells become naked or hidden singles, and which cells the early eliminations land on.
Same total puzzle space as classic, but the diagonal logic is what unlocks them. If you're a classic player, X-Sudoku rewards thinking about the diagonals before the boxes.
Stillgrid plays X-Sudoku at 6×6, 9×9, and 16×16 — a playable 16×16 diagonal sudoku is nearly impossible to find anywhere else.
Rules
- All classic rules: rows, columns, 3×3 boxes each contain 1–9.
- The top-left to bottom-right diagonal contains each digit 1–9 exactly once.
- The top-right to bottom-left diagonal contains each digit 1–9 exactly once.
How Stillgrid plays
- Diagonal cells visually highlighted so you don't forget the extra constraint.
- Pencil marks, auto-pencil, undo/redo.
- Difficulty graded against the diagonal-aware solver.
- Free to play. No signup needed to play.
Common questions
What is X-Sudoku?
X-Sudoku, also called diagonal sudoku, is a sudoku variant that adds two constraints to the classic 9×9 rules: each of the two main diagonals must also contain the digits 1 to 9 exactly once.
How is X-Sudoku different from classic sudoku?
Every classic rule still applies, but the two main diagonals become unique units of their own. Cells on the diagonals carry more constraints, which changes the order in which cells resolve and where the early eliminations land.
Can I play X-Sudoku at 16×16?
Yes. Stillgrid plays X-Sudoku at 6×6, 9×9, and 16×16. The 16×16 diagonal grid uses digits 1–9 plus letters A–G, and both main diagonals must contain all sixteen symbols.
Is X-Sudoku free on Stillgrid?
Yes. X-Sudoku and every other variant are free to play, with no signup needed to start a puzzle.