--- title: "Do Diagonals Count in Sudoku? Sudoku X Rules | Sudoku Flow" description: "In classic Sudoku the diagonals do not count. In Diagonal Sudoku, or Sudoku X, both long diagonals must also hold 1 to 9: the rules, an example and tips." url: https://sudokuflow.com/guides/diagonal-sudoku-rules/ language: en last_modified: 2026-11-05 --- [Sudoku guides](https://sudokuflow.com/guides/) # Do the Diagonals Count in Sudoku? Diagonal Sudoku (Sudoku X) Rules In classic Sudoku the diagonals do not count. In Diagonal Sudoku, or Sudoku X, both long diagonals must also hold 1 to 9: the rules, an example and tips. Updated 2026-11-05 ## Quick answer Not in classic Sudoku: only rows, columns and 3×3 boxes must hold 1 to 9, and a diagonal may repeat a digit. Diagonal Sudoku, also called Sudoku X, adds one rule: each of the two long diagonals must hold 1 to 9 as well. That gives you two more units to work with. - Classic Sudoku: rows, columns and boxes only. - Sudoku X: both long diagonals also hold 1 to 9. - The centre square sits on both diagonals. - A digit can be placed because of a diagonal alone. - Without the diagonal rule, our example would have 92 answers. ## Classic Sudoku: the diagonals are free The classic rule names three kinds of units: rows, columns and 3×3 boxes. The diagonals are not among them, so a finished classic Sudoku can repeat a digit on a diagonal, and no step in a classic puzzle may use them. ## Diagonal Sudoku: two more units In Diagonal Sudoku the two long diagonals, from corner to corner, are units too: each holds every digit from 1 to 9 once. Everything else stays the same. Together the two diagonals form an X across the grid; the variant is also called Sudoku X. ## An example The puzzle below has exactly one answer under the diagonal rule. Without that rule, the same 30 clues would allow 92 different answers, so the diagonals are not decoration: you need them to solve it. The shaded squares are the two diagonals. The shaded squares are the two diagonals.: ```text . . . | . . 7 | . 6 2 . . . | 1 2 6 | . . 3 . 7 2 | 3 8 . | . . . ------+-------+------ . 8 . | . . . | 2 5 6 4 . . | . . 9 | 1 . . 7 . . | . . . | . . . ------+-------+------ . . 7 | 8 . 3 | 6 . 1 . . . | 9 . 2 | . 4 7 . . . | . 7 . | . 2 . ``` ## Tips for the diagonals - Treat each diagonal like a row: check which digits it is still missing. - A digit with only one possible square left on a diagonal goes there, just as in a row. - The centre square is on both diagonals, so it is one of the most constrained squares on the grid. - Each corner box holds three squares of one diagonal, and the centre box holds five diagonal squares, one of them shared. 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