Forced cell on the diagonal
A Sudoku X (Diagonal Sudoku) deduction: a cell on the diagonal whose digit is forced by row, column, box, and diagonal together — no three of them suffice.
A forced cell on the diagonal is a deduction specific to Sudoku X: a cell sitting on one of the two diagonals where the row, column, box, and diagonal constraints together leave exactly one candidate, even though no three of those constraints would have. The deduction has no equivalent in classic Sudoku because classic Sudoku doesn't have the diagonal as a fourth unit.
Interactive example
Step 0 / 6
When it fires
Consider a cell on a diagonal at row 4, column 4. Its row already contains some digits; its column contains others; its box contains a third set. If those three units together leave two candidates standing — call them 6 and 9 — and the diagonal already contains a 9 somewhere else, the diagonal eliminates the 9 from row 4, column 4 and the cell becomes a forced 6.
In a classic Sudoku at the same configuration, the cell would have remained two-candidate and the solver would need a naked pair or a pointing pair elsewhere to make progress. The diagonal collapses the deduction to a single step.
How to spot it
The visual cue on Sudoku Mountain's board is the dotted ring around the 17 cells participating in the diagonal constraint. When you find yourself working on a cell that bears the ring and the row-column-box scan has left two or more candidates, the diagonal is the unit to check next. The deduction is identical in shape to a naked single — it just runs over four units instead of three.
This is the signature move of the variant. Easy Sudoku X puzzles include it two or three times; medium has three to five; hard and expert make it central to the solve.
For the practical-tactic article, see forced diagonal cells.
See also
- Sudoku X— A Sudoku variant where both diagonals must also contain each digit 1-9 exactly once. Also known as Diagonal Sudoku. Classic Sudoku plus two new units.
- Diagonal constraint— The extra rule that defines Sudoku X (Diagonal Sudoku): each of the two diagonals must contain every digit 1-9 exactly once, just like a row or column.
- Diagonal pair— A naked pair where both cells lie on the same diagonal of a Sudoku X. Eliminates the two pair digits from every other cell on that diagonal.
- Naked single— A cell on the Sudoku grid that has only one legal candidate left — the simplest deduction in the game, and the one that solves most of an easy puzzle.
- Hidden single— A digit with only one possible cell within a unit (row, column, or 3×3 box) — even if that cell could legally hold other digits. The unit-first sibling of the naked single.
Read more
- Forced diagonal cells
The signature technique of Sudoku X / Diagonal Sudoku — a cell on the X where row, column, box, and diagonal together force the digit, even when no single constraint does.
- When the diagonal helps
A practical tactic for Sudoku X (Diagonal Sudoku): scan the diagonal when a row-column-box pass stalls. Worked examples of where the extra unit pays off.
- The rules of Sudoku X
Sudoku X — also called Diagonal Sudoku — is classic Sudoku with one extra rule: both diagonals must also contain each digit 1–9 exactly once.