Sudoku X (also called Diagonal Sudoku) keeps every Classic rule and adds the two corner-to-corner diagonals as houses. Each diagonal holds 1 to 9 exactly once, just like a row. That gives you more information, and a new place to look for every technique you already know.
The rules
Fill the 9×9 grid so that every row, every column and every 3×3 box holds the digits 1 to 9 exactly once. That part is plain Classic Sudoku.
Then add the two long diagonals: the main diagonal from the top-left corner (R1C1) to the bottom-right (R9C9), and the anti-diagonal from the top-right corner (R1C9) to the bottom-left (R9C1). Each one must also hold 1 to 9 exactly once. On the board they are shaded and drawn with a fine line, so you can always see which cells they run through.
The centre cell, R5C5, sits on both diagonals at once. Its digit counts for both.
What the diagonals change
A cell off the diagonals sees 20 other cells, the same as in Classic. A cell on one diagonal sees 26, and the centre cell sees 32. More cells in sight means fewer candidates, so diagonal cells tend to resolve early.
The main diagonal runs through boxes 1, 5 and 9; the anti-diagonal through boxes 3, 5 and 7. In each of those boxes the diagonal takes exactly three cells, and box 5 holds five diagonal cells, because the two diagonals cross in it.
The diagonals do real work. Across the 4,000 puzzles on this site, about one deduction in five uses a diagonal as the deciding house.
How to solve it
Start with the diagonals. Count which digits each one already holds; a diagonal with six or seven digits placed is often the quickest single on the board.
Watch the centre and the four corners. Each sits in four or five houses, so a digit placed elsewhere on a diagonal can be the one that pins it.
When a digit in box 1, 3, 5, 7 or 9 is confined to that box’s diagonal cells, it is locked: it cannot appear anywhere else on that diagonal. The reverse also holds. These box–diagonal locks are the main new idea at Medium.
Worked examples
Three positions from real puzzles on this site, walked through by the same Next Step teacher you get in a game. Step through each one.
The diagonal settles a single
Look at row 2. One empty cell here has only one candidate left.
A hidden single on a diagonal
Look at the anti-diagonal (from R1C9 to R9C1). In Sudoku X it needs every digit from 1 to 9, just like a row. Where can 6 go on it?
A box and a diagonal lock a digit
In box 5, find every cell that can still take 8.
The four levels
Sudoku X levels are graded on the same scale as Classic, so a Medium Sudoku X needs the same techniques as a Medium Classic puzzle. Every puzzle is solvable by logic alone, which is why there is no Diabolical level.
Classic techniques, with a diagonal added
Every Classic technique still works. Most gain a diagonal version, because a diagonal is a house like any other. These lessons teach them on Classic boards:
Frequently Asked Questions
Is Sudoku X harder than Classic Sudoku?
Not by itself. The extra rule gives you more information, not less. Levels here use the same technique scale as Classic, so a Hard Sudoku X needs the same techniques as a Hard Classic puzzle. What changes is where you look.
Do both diagonals have to hold 1 to 9?
Yes. Both long diagonals, corner to corner, must each hold every digit once. Shorter diagonals have no rule.
What about the centre cell, where the diagonals cross?
R5C5 belongs to both diagonals, plus its row, column and box: five houses in all. It holds one digit, and that digit counts for every one of them.
Do Classic techniques still work?
All of them. Singles, locked candidates and subsets also work on a diagonal, and an XY-Wing can link cells along one. X-Wing and Swordfish stay row and column patterns.
Why is there no Diabolical level?
Every Sudoku X puzzle on this site can be solved by logic, up to Extreme techniques. Diabolical puzzles need trial and error, so they are left out.
Why is it called Sudoku X?
The two diagonals cross the grid in an X. It is also known as Diagonal Sudoku.
Which tools can I use?
All of them: pencil marks in corner or centre notation, candidates, Auto Check, Check Cell and Check Puzzle, and Next Step, which names the diagonal whenever it is the reason for a move.
Try one now. Easy is a good place to start.