- Jigsaw Sudoku replaces the 3×3 boxes with nine irregular, connected regions of 9 cells each
- Diagonal Sudoku adds a rule: each main diagonal (top-left to bottom-right and vice versa) must also contain 1–9 once
- Both variants use the same core rule — only the unit shapes change
- Jigsaw regions can span multiple rows and columns, creating very different constraint patterns
- Diagonal Sudoku often has fewer given digits because the diagonals add significant extra constraints
Jigsaw (Irregular) Sudoku
In standard Sudoku, the nine 3×3 boxes are square. In Jigsaw Sudoku (also called Irregular Sudoku), the nine regions are irregular shapes — each still contains exactly 9 cells, but the shapes can zigzag, extend diagonally, or span multiple standard-box positions.
The core rule is unchanged: each row, each column, and each region must contain digits 1–9 exactly once. But because regions no longer align with the 3×3 grid structure, cross-hatching works differently. You must track region boundaries carefully rather than relying on visual box groupings.
Solving Jigsaw Sudoku
All standard techniques apply — but visually identifying which cells belong to each region is the primary challenge. Tips:
- Color-code or outline regions before solving
- Treat each region as you would a box: scan for digits that can only go in one cell within the region
- Be careful with locked candidates — region-to-line interactions still work, but the region shapes make them less predictable
Diagonal Sudoku
Diagonal Sudoku adds two extra constraints beyond standard rules: the main diagonal (top-left to bottom-right) and the anti-diagonal (top-right to bottom-left) must each contain digits 1–9 exactly once.
This creates two more units — bringing the total from 27 to 29. The intersection cells (particularly the center cell R5C5, which belongs to both diagonals, its row, its column, and its box) are among the most constrained cells in any Sudoku variant.
Using Diagonal Constraints
When a digit appears on the main diagonal elsewhere, eliminate it from all other diagonal cells in the same diagonal — treat the diagonal exactly like a row or column. Diagonal cells at the corners (R1C1, R1C9, R9C1, R9C9) each belong to one diagonal, one row, one column, and one box — four units simultaneously.
Diagonal Sudoku puzzles typically have fewer given digits (often 20–26) because the extra constraints allow the puzzle to be uniquely solvable with less information. Try a standard Easy puzzle first to get comfortable with the core rules before exploring variants.