Sudoku-YUnderstanding the game
Diagonal Sudoku: two more no-repeat lines
Diagonal Sudoku keeps the familiar rules and gives you two more directions to inspect.
What changes from Classic
On a 9×9 Diagonal or Sudoku X board, rows, columns and boxes still contain 1–9 once each. The main diagonals A1–I9 and A9–I1 also each contain 1–9. Check the maker’s marked lines and rules before solving a variation.
Our example: two diagonal gaps
Suppose A1–I9 has seven filled cells and is missing 2 and 7. Its only gaps are E5={2,7} and I9={7}. Place I9=7, then E5=2. The second deduction uses the diagonal: these cells share no ordinary row, column or box.
If I9 could also contain 2, the pair of gaps alone would not establish their order. The extra single is what starts the deduction in this partial teaching example.
The middle belongs to both lines
E5 sits on both main diagonals. A placement there must satisfy both diagonals as well as its ordinary units. If you add the two diagonal totals for a later sum argument, remember that E5 was counted twice. Two crossing lines do not make two separate central cells.
Reuse reasoning, not unstated rules
Singles and candidate pairs can still use the idea of one occurrence per unit; there are simply extra units to inspect. A deduction relying on a diagonal, however, cannot be transferred to a Classic puzzle without that rule.
Sudoku-Y currently offers Classic and Killer. This variation is not yet a playable mode here. Always read the puzzle maker’s rules before applying a technique.
Try before looking
Try before revealing
In ordinary Classic, does A1=6 rule out 6 at E5 because they share a diagonal?
Show explanation
No. Classic has no diagonal unit, and those two cells share no ordinary row, column or box.
References and further reading
Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.