X-Sudoku: standard rules + both diagonals 1–9 — free online or as a book
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Fill every cell with 1–9: rows, columns, 3×3 boxes, and both shaded diagonals each contain 1–9 once. Tap a cell, then tap a number.
How Diagonal Sudoku works
Diagonal Sudoku (X-Sudoku) adds one rule to standard Sudoku: both main diagonals (top-left to bottom-right and top-right to bottom-left) must each contain the digits 1 to 9 exactly once. The diagonal cells are highlighted on the board. Every puzzle has exactly one solution reachable by pure logic.
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Prefer paper?
Large Print Diagonal Sudoku — 110 X-Sudoku puzzles (Easy to Hard), as a large-print paperback (also in German).