Diagonal Sudoku Strategy: How to Solve X-Sudoku
Diagonal Sudoku — also called X-Sudoku — adds two extra constraints to the classic grid: both main diagonals must also contain digits 1–9 with no repeats. Here's how to use that extra rule to your advantage.
1. Understand the diagonal constraint
On top of the standard row, column, and 3×3 box rules, every cell on the two main diagonals belongs to an extra 9-cell group. That means each diagonal cell has up to 4 constraints acting on it (row, column, box, diagonal) instead of 3 — a huge solving advantage if you scan the diagonals first.
2. Start at the diagonals, not the corners
The two diagonals cross at the center cell (r5c5), which sits under five constraints at once. Always check it first. After that, scan each diagonal end-to-end for naked singles before touching the rest of the grid — solved diagonal cells cascade quickly into rows, columns, and boxes.
3. Cross-hatch with the diagonal in mind
When placing a digit by cross-hatching, treat each diagonal as a third elimination line. If a digit already appears on a diagonal, remove it from every other diagonal cell. This usually unlocks placements that a classic-sudoku scan would miss.
4. Naked and hidden pairs on the diagonal
Naked pairs and hidden pairs work on the diagonal exactly as they do on any row, column, or box. Two cells on the same diagonal sharing only the same two candidates lock those digits to that pair and remove them from the other seven diagonal cells.
5. Pointing pairs across a diagonal
If a candidate inside a box is restricted to cells that all sit on the same diagonal, that candidate can be eliminated from every other cell on the diagonal — a diagonal-aware version of the classic pointing-pair technique.
6. Advanced: X-wing and coloring on diagonals
For expert X-Sudoku puzzles, X-wing and simple coloring still apply, but you can also build chains that step from a row or column onto a diagonal. Whenever a strong link lands on a diagonal cell, follow the diagonal to look for additional eliminations.
7. Common mistakes
The most frequent X-Sudoku error is forgetting to eliminate against the diagonal after placing a digit. Build a habit: place, then sweep the diagonals it touches (if any). The second most common mistake is double-counting the center cell — it belongs to both diagonals, so a digit placed there removes itself from 16 other cells, not 8.
Keep practicing
Diagonal Sudoku rewards solvers who treat the diagonals as first-class constraints. Drill the techniques in solo mode and combine them with the patterns from our hard puzzles guide to break through expert-level X-Sudoku.