A LITTLE FOCUS. A FRESH START.
Play Diagonal Sudoku (Sudoku X)
Play Diagonal Sudoku, also called Sudoku X, free online. Solve both marked diagonals alongside the usual rows, columns, and boxes.
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LEARN THE TWIST
How to play Diagonal Sudoku (Sudoku X)
Unlike Classic, both corner-to-corner diagonals must also contain 1–9 without repeats. Other, shorter diagonal lines have no special rule.
The rules
Fill each row, column, and 3×3 box with 1–9, without repeats. Both marked main diagonals also contain 1–9 without repeats. Also called Sudoku X.
A useful first move
Treat each marked diagonal like an extra row. Check its missing digits whenever a new number lands on it, especially in the center of the grid.
A 6 on the top-left to bottom-right diagonal rules out 6 in every other cell on that diagonal. The center cell belongs to both main diagonals, so a digit there constrains both.
SOLVE IT STEP BY STEP
Use a distant digit on the same diagonal
A marked main diagonal is an extra nine-cell unit. Its cells can rule out digits even when they share neither a row nor a column nor an ordinary box. The center is especially useful because it belongs to both marked diagonals.
- Center cell
- 8
Identify the extra shared unit.
Row 1, column 1 and row 5, column 5 both lie on the top-left to bottom-right main diagonal.
Remove the repeated diagonal digit.
The diagonal already contains 6. Remove 6 from the center’s candidates, even though ordinary row, column, and box checks left it available.
Check both diagonals after placing.
The center is 8. That placement now excludes 8 along both main diagonals, as well as in its ordinary row, column, and box.
Scan a diagonal for a hidden single
List a diagonal’s missing digits, then check where each could still fit. If only one diagonal cell accepts a 4, that cell is 4 even when its pencil marks also contain other numbers. The diagonal must contain its own 4 exactly once.
Common mistakes to avoid
- Only the two marked corner-to-corner diagonals are extra units. Short parallel diagonals have no special rule.
- Being on one diagonal does not put a cell on the other. The center is their only intersection.
Try the reasoning yourself
The center contains 2. Where does the diagonal rule remove 2?
Show the answer and why
From every other cell on either marked main diagonal. The ordinary rules also remove it from the center’s row, column, and box. A cell outside those units is not affected by this placement alone.
Controls, difficulty, and saved games
Choose Beginner, Easy, Medium, Hard, or Expert above. Select an empty cell, then tap a number or use your keyboard. Notes let you pencil in candidates; three hints are available. Three mistakes end the puzzle.
Every mode and difficulty keeps a separate save in this browser. You can switch puzzles and return to your place. Clearing site data removes these saves; they do not sync with another device or the iPhone app. Settings include night mode and optional automatic cell advancement.
Correct entries earn points. High scores are stored by mode and difficulty and update only when beaten; completed-puzzle count and day streak are shared.
FIFTEEN WAYS TO PLAY
Explore all Sudoku modes
A familiar grid, a different kind of logic. Every mode is free to play in your browser.