A twist with teeth
Sudoku X (also called Diagonal Sudoku) keeps the familiar 9×9 grid and adds a single new rule: both long diagonals — top-left to bottom-right, and top-right to bottom-left — must also contain 1–9 with no repeats.
It sounds minor. It isn't. The diagonal constraint propagates information from corner to corner of the grid, which means techniques that rarely trigger in classic Sudoku (like X-Wing on diagonals, or diagonal pointing pairs) suddenly become essential. The puzzle feels both familiar and fresh.
When to play it
Sudoku X is the perfect next step for solvers who've mastered Classic and want harder logic without learning a brand-new ruleset. Difficulty is graded separately: a Sudoku X rated "Medium" is usually harder than a Classic "Hard", because the diagonal constraints make puzzle design tighter and the solving paths more intricate.
How to play
- The grid is the same 9×9 you already know.
- All classic Sudoku rules apply — rows, columns, and 3×3 boxes.
- New rule: the main diagonal (top-left → bottom-right) must contain 1–9 with no repeats.
- New rule: the anti-diagonal (top-right → bottom-left) must contain 1–9 with no repeats.
- Solve as usual — but the diagonals give you extra starting information.
Strategy tips
- Work the corners early. Diagonal constraints hit the four corner cells hardest, so they tend to lock down quickly.
- Watch diagonal singles. A digit that appears only once on a diagonal is a free placement.
- Cross-check candidates. When eliminating, remember cells on a diagonal have one extra group to respect.
- Beware of duplicate solutions. Badly designed Sudoku X puzzles can have multiple solutions — play ones labelled "unique solution guaranteed".