Sudoku X Puzzles
Classic sudoku plus one extra rule: both main diagonals must also contain the digits 1 to 9. Sixty verified grids from gentle to tough, each one with a unique solution and every deduction explained.
Also known as Sudoku X, diagonal sudoku and Sudoku with diagonals. The classic variant is on the sudoku page; the cage version is on the killer sudoku page.
Classic Sudoku Plus Two Diagonals
Sudoku X keeps every classic rule and adds one: both main diagonals must contain the digits 1 to 9 exactly once. The diagonals cross at the centre cell, so the centre belongs to a row, a column, a box and both diagonals at once — the most constrained square on the board.
How To Solve Sudoku X
- Solve it like classic sudoku first. Scanning, cross-hatching and candidate elimination work unchanged.
- Use the diagonals as extra groups. Any digit already placed on a diagonal disappears from the rest of that diagonal.
- Watch the corners and the centre. The four corners sit on a diagonal; the centre sits on both. They carry the strongest deductions.
- Audit the X before answering. A grid where every row, column and box is correct can still fail on a diagonal.
The centre cell is the key square. It sits on both diagonals and in the centre box, so a digit placed there removes more candidates than any other placement. Start every deduction that involves it.
The Eight Grid Families
| Family | About | Character |
|---|---|---|
| Learning the X | 33–36 clues | Gentle grids where the X is easy to use |
| Steady X grids | 31–32 clues | Hidden singles start to matter |
| Classic symmetry | 30 clues, mirrored | The newspaper look with the X rule |
| Scanning practice | 29 clues | Digit hunting across the diagonals |
| Fewer givens | About 28 clues | The X has to work harder |
| Digit hunting | 27 clues | Long chains across both diagonals |
| Lean grids | 26 clues, free-form | No symmetry safety net |
| Sparsest here | 26 clues, mirrored | The toughest X grids generated |
From Sudoku X To The Other Variants
Sudoku X is the gentlest variant because it keeps the regular 3×3 boxes. The other variants change more: killer sudoku adds cages with sums, and jigsaw sudoku replaces the boxes with irregular shapes entirely. If the X grids feel comfortable, both are natural next steps — killer first, jigsaw after.
Knights & Knaves Puzzles: Frequently Asked Questions
What is Sudoku X?
Sudoku X is classic sudoku with one extra rule: both main diagonals must also contain the digits 1 to 9 exactly once. Every row, column, 3×3 box and diagonal holds all nine digits.
How is Sudoku X different from classic sudoku?
Only by the two diagonals. Every row, column and box rule is the same, but the cells along both main diagonals belong to an extra group, which removes candidates from them and adds a constraint the classic puzzle does not have.
How do you solve Sudoku X?
Solve it like classic sudoku — scanning, cross-hatching, candidate elimination — but audit the two diagonals constantly. The diagonal cells are the ones the X rule constrains, so they give the most extra deductions.
Do the diagonals always help?
Yes. Any digit already on a diagonal removes itself as a candidate from the rest of that diagonal. Since the diagonals cross the whole grid, they usually resolve cells that row and column logic alone cannot.
How many puzzles are there?
There is no fixed number. Each grid is rebuilt from the short seed in the page address, so the supply is effectively unlimited. Copying the grid link saves that exact grid permanently.
Can I practise one pattern at a time?
Yes. Pick any pattern from the catalogue above and the board switches to that grid only, with its own address you can bookmark or hand out.
Is Sudoku X harder than classic sudoku?
Slightly. The X rule adds constraints, so fewer placements are possible at any point — but those extra constraints also create deductions that classic sudoku lacks. Most solvers find a Sudoku X grid similar to a classic grid of the same clue count.
Are the corners special in Sudoku X?
Yes. All four corner cells sit on a diagonal, and the centre cell sits on both. Corner and centre deductions are stronger than ordinary ones, which is the first place to look when stuck.