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

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.

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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.

Sudoku variants: Killer, Jigsaw, Diagonal, Samurai and more →

Practice

Every Find the Mistake puzzle on this site has exactly one wrong digit, and you can play each board right on its page.

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