X Sudoku 4×4 Hard — Set 21
Solver Notes
X-Sudoku adds diagonal constraints to classic sudoku. Both main diagonals must each contain digits 1-9, creating extra deduction opportunities that make this the most accessible sudoku variant for beginners.
What is X-Sudoku?
X-Sudoku (also called Diagonal Sudoku) adds two diagonal constraints to standard 9×9 sudoku. The main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each contain each digit exactly once, in addition to the standard row, column, and box constraints.
How to Play X-Sudoku
- Fill the grid so each row, column, and 3×3 box contains digits 1-9 exactly once
- Additionally, both main diagonals must contain digits 1-9 exactly once
- Use the highlighted diagonal cells as an additional constraint to narrow possibilities
Grid Sizes Available
- 4×4 — Great for beginners and quick sessions (30 sec – 2 min)
- 6×6 — Intermediate challenge with 2×3 boxes (3–10 min)
- 9×9 — Full standard size with diagonal constraints (10–60 min)
Why X-Sudoku is Perfect for Beginners
Unlike Killer or Jigsaw, X-Sudoku uses only one type of extra rule — the diagonal constraint. This single addition is easy to understand and actually helps narrow solutions faster than standard sudoku, making it an ideal first variant to try.
What makes X-Sudoku different from regular sudoku?
X-Sudoku adds two diagonal constraints: both main diagonals of the 9×9 grid must each contain digits 1-9 without repetition. This gives you extra deduction power and makes the puzzle visually distinct.
Is X-Sudoku harder than regular sudoku?
At the same difficulty rating, X-Sudoku is often easier than regular sudoku because the diagonal constraints give you more information. However, Expert and Extreme X-Sudoku puzzles can be very challenging.
Can I print X-Sudoku puzzles?
Yes. Every X-Sudoku puzzle on FreeSudoku has a print option. The printable version shows the diagonal cells highlighted so you can track the constraint on paper.