Sudoku variant
Sudoku X
Also called Diagonal Sudoku. Classic 9×9 with one extra rule: each diagonal also carries the digits 1 through 9.
How Sudoku X differs from Classic
Sudoku X — also called Diagonal Sudoku — plays exactly like Classic Sudoku. Same 9×9 grid, same digits 1–9, same row, column, and 3×3 box uniqueness. The only addition is that both diagonals — top-left to bottom-right, and top-right to bottom-left — each carry the same uniqueness constraint, so every digit 1 through 9 appears once on each diagonal.
There are seventeen X-cells in total: nine on each diagonal, with the centre cell shared between them. On the board, the diagonal cells are marked with a dotted coral ring so you can see at a glance which placements participate in the extra constraint.
In practice the diagonal rule helps more than it hurts. Once a few digits land on a diagonal, the candidate set for the remaining diagonal cells often shrinks fast. When row, column, and box scanning stalls, the diagonal is the unit that breaks the puzzle open — a hidden single that the standard three units couldn't see often falls out the moment you look across the X.
Tier calibration mirrors the published Sudoku X apps: Easy carries 36 givens, Medium 34, Hard 28, Expert 24. Every puzzle is uniqueness-gated against the diagonal rule, so every X tier guarantees one and only one solution.
Quick rules
- Every row contains each digit 1–9 exactly once.
- Every column contains each digit 1–9 exactly once.
- Every 3×3 box contains each digit 1–9 exactly once.
- Both main diagonals contain each digit 1–9 exactly once.
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.
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.
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.
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.
A short history of Sudoku X
Where Sudoku X (Diagonal Sudoku) came from, how it spread alongside the 2005 Sudoku boom, and what its quietly steady following looks like today.
Sudoku X for beginners
Your first Sudoku X puzzle, walked through end to end — also called Diagonal Sudoku. What to scan for, when the diagonal helps, and how to know you're ready for medium.