About Diagonal Sudoku (Sudoku X)

Sudoku X adds one rule to the classic game: both long diagonals, corner to corner, must also contain the digits 1 to 9 exactly once. The two shaded diagonals on the board mark them. Everything else is ordinary sudoku — rows, columns and 3x3 boxes behave exactly as they always do.

That single addition changes the solving experience more than it sounds like it should. Twenty-one of the eighty-one cells sit on a diagonal, and the centre cell sits on both, so it answers to four separate units at once and is usually the most constrained square on the grid. The extra constraints also mean these puzzles need fewer clues to stay uniquely solvable — an expert grid opens with twenty-two — so the board looks emptier than a classic sudoku of the same difficulty while actually being no harder.

The reward is that the diagonals give you a way in when ordinary scanning dries up: a diagonal crosses every box on its path, so it carries information between regions that share neither a row nor a column.

Tips to win

• Start on the diagonals: they are the most constrained units on the board and usually the only ones that open early
• The centre cell belongs to a row, a column, a box and both diagonals — solve it and four units narrow at once
• When a classic scan stalls, look for a digit missing from a diagonal; it often places somewhere a row or box scan cannot reach
• Do not read the sparse grid as harder: fewer clues is what the extra rules buy, not a difficulty increase