Diagonal Sudoku (X-Sudoku): rules & strategy
Learn how the two diagonals change the puzzle, pick up strategy tips, then play free Diagonal Sudoku online.
What is Diagonal Sudoku?
Diagonal Sudoku, also called X-Sudoku, is regular Sudoku with two extra rules. The two long diagonals of the grid, from corner to corner, must each contain the digits 1 to 9 exactly once. The usual row, column and 3×3 box rules still apply.
The two diagonals cross at the centre cell, which makes that cell one of the most constrained in the whole puzzle.
The rules in short
- Each row contains 1–9 once.
- Each column contains 1–9 once.
- Each 3×3 box contains 1–9 once.
- The main diagonal (top-left to bottom-right) contains 1–9 once.
- The anti-diagonal (top-right to bottom-left) contains 1–9 once.
How it differs from Classic
Every cell on a diagonal now belongs to four groups instead of three (and the centre cell to five), so it has fewer candidates. That gives you extra elimination chances, but it also means a normal Sudoku strategy can miss moves that the diagonals make obvious.
Strategy tips
- Scan the diagonals first. A digit missing from a diagonal with only one place left is a hidden single.
- Check the centre cell early. It sits in its row, column, box and both diagonals, so few candidates fit.
- When a digit is already on a diagonal, cross it off every other cell on that diagonal.
- Use pencil notes on diagonal cells, since they carry the most eliminations.
- Fall back on the standard techniques in our Sudoku tips guide.
Try it
Play a free Diagonal Sudoku puzzle at Easy, Medium or Hard, with hints that understand the diagonal rule. You can also read about Killer Sudoku, or compare all Sudoku modes.
Play Diagonal Sudoku