Diagonal Sudoku.
Learn the rules, see the possibilities.
Diagonal Sudoku, also called Sudoku X, adds two extra rules to the classic puzzle. Both corner-to-corner diagonals must contain the complete symbol set without repeats. The highlighted X gives you two more groups to use when rows, columns, and boxes are not enough.
How to play Diagonal Sudoku
Three ideas. Illustrated on the board. Take them one move at a time.
Follow the first diagonal
Trace the highlighted cells from one corner to the opposite corner. On a 9×9 board, these nine cells must contain 1–9 exactly once. A digit already present on this line cannot appear again anywhere along it, even when the cells are in different boxes.
Only the full corner-to-corner diagonal is an extra group, not every slanted line.
Check the other arm of the X
The opposite diagonal runs from the top-right corner to the bottom-left. Treat it as its own group, not as part of one combined 18-cell region. The same digit can appear once on each diagonal, provided its placements also satisfy the normal Sudoku rules.
When considering a cell, check only the extra diagonal or diagonals that actually pass through it.
Use the center as a meeting point
On an odd-sized board such as 9×9, both main diagonals meet in the center cell. This cell must satisfy its row, column, box, and both diagonals. Eliminating candidates from all five groups can make the center especially useful. Other diagonal cells usually belong to only one arm of the X.
Even-sized boards have no single center cell shared by both diagonals.
Sudoku strategies that work in every variant
Every Sudoku is solved with the same three habits. They need no arithmetic and no guessing — just a careful look at what each cell can still be.
Scan rows, columns, and boxes
Pick a number and sweep the grid. Where a number already appears, it cannot go again in that row, column, or box. Three placed copies often leave only one open cell for the fourth.
Hunt for singles
Look for a cell with just one possible symbol left. A row or box that is almost complete is the fastest place to find one, and each single you place narrows the next cell.
Write candidates, then eliminate
When two or three cells stay open, switch on Notes and pencil in the possibilities. Cross out candidates as peers fill in. If a digit fits only one cell inside a group, that cell must hold it.
A few Diagonal Sudoku facts
Extra all-different groups
One for each corner-to-corner diagonal.
Distinct diagonal cells
On a 9×9 board, the two diagonals share the center.
Another familiar name
Sudoku X refers to the shape of the two extra diagonals.
These examples and figures describe a standard 9×9 board unless otherwise stated.
Make yourself at home on the board.
Choose a cell, enter a symbol, and keep going. Use Notes for possibilities, Undo to revisit a move, and Hint when you need a nudge. Change the visual theme without changing your game.
- ↑↓←→
- Move between cells
- 1–9
- Enter a number; use the keypad for larger symbol sets
- Del
- Erase your entry
- Ctrl / ⌘Z
- Undo the last move
Diagonal Sudoku, explained.
A few things you might be wondering.
Do all diagonal lines need different numbers?
No. Only the two main diagonals running from corner to opposite corner are extra groups. Other slanted lines have no special no-repeat rule.
Do normal Sudoku rules still apply in Sudoku X?
Yes. Each row, column, and box must still contain the full symbol set without repeats. The diagonal constraints are additional rules, not replacements.
Is Diagonal Sudoku always harder than Classic Sudoku?
Not necessarily. The extra constraints can rule out candidates and create useful deductions. Difficulty depends on the starting clues and the reasoning needed to complete the particular puzzle.