The case notes
How the anti-king rule works
Cells a chess king's move apart cannot hold the same digit, including diagonals. Watch a worked example, then try the idea in a puzzle.
Put it into practice
Equal digits cannot touch
In an anti-king Sudoku, equal digits cannot touch, even at a corner. This five sits at the edge of a box.
The diagonal counts
Look one step diagonally up and left. A king could move there in a single step, so another five cannot.
Across the box border
These two cells are in different rows, columns and boxes. Normal Sudoku does not connect them. The anti-king rule does.
All eight neighbours
Check every cell touching this five: above, below, beside it and diagonally. None may contain another five.
No matching neighbours
Keep the usual Sudoku rules. Add this check whenever you place a digit: the same digit cannot touch it.
Read the transcript
In an anti-king Sudoku, equal digits cannot touch, even at a corner. This five sits at the edge of a box. Look one step diagonally up and left. A king could move there in a single step, so another five cannot. These two cells are in different rows, columns and boxes. Normal Sudoku does not connect them. The anti-king rule does. Check every cell touching this five: above, below, beside it and diagonally. None may contain another five. Keep the usual Sudoku rules. Add this check whenever you place a digit: the same digit cannot touch it.
