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.
Sudoku X is a Sudoku variant played on the standard 9×9 grid with one additional constraint: both diagonals must also contain each of the digits 1 through 9 exactly once, like a row or a column. The two diagonals — top-left to bottom-right, and top-right to bottom-left — trace an X across the board, which is where the variant gets its name. The puzzle is also called Diagonal Sudoku, and the two names are used interchangeably.
How it differs from classic
Every classic Sudoku rule is preserved. The differences are: each cell on the diagonal running from (1, 1) to (9, 9) belongs to a fourth unit (that diagonal) in addition to its row, column, and box. Each cell on the diagonal running from (1, 9) to (9, 1) similarly belongs to its diagonal as a fourth unit. The centre cell at (5, 5) lies on both diagonals.
Seventeen cells in total participate in the diagonal constraint — nine on each diagonal, minus the centre cell that's shared between them.
How it plays
The technique vocabulary is unchanged from classic. Every move — naked single, hidden single, naked pair, locked candidates, X-wing — works identically; the diagonal just gives those moves a fourth unit to fire on for cells inside it. The one move that's genuinely specific to the variant is the forced cell on the diagonal: a cell whose digit is forced only when row, column, box, and diagonal are taken together.
For a hands-on introduction, Sudoku X for beginners walks through a first puzzle end to end, and when the diagonal helps covers the practical tactics built on the variant's extra unit.
See also
- Diagonal Sudoku— An alternate name for Sudoku X — the 9×9 Sudoku variant where both diagonals carry the same digit-uniqueness rule as rows, columns, and boxes.
- 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.
- 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.
- 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.
- Unit— Collective name for a row, column, or 3×3 box — the three groupings Sudoku's no-repeats rule applies to. Every cell sits in exactly three: its row, column, and box.
Read more
- 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.
- Meet Sudoku X
An introduction to Sudoku X (a.k.a. Diagonal Sudoku) — what the diagonal rule adds, why classic solvers tend to enjoy it, and how to know if it's for you.
- How Sudoku X differs from classic
For solvers who already play classic Sudoku — what changes mentally when you switch to Sudoku X (Diagonal Sudoku), and what stays exactly the same.