sudokUI › Sudoku techniques › Tridagon
Tridagon in sudoku
Four boxes forming a rectangle each hold three cells on a diagonal, one box slanting against the other three; only one cell, the guardian, has candidates besides X, Y, Z.
Why it works
Twelve such cells can never all be filled from X, Y and Z without repeating a digit in some row, column or box. So the guardian must take one of its other candidates, and X, Y and Z are eliminated from it.
How to spot it
Look for four boxes crowded with the same three candidates; diagonals may wrap round a box edge, stepping right or left as they go down.
Only ever needed in puzzles made by hand for it: none can be generated, so it is not offered in practice.
In a puzzle's rating
Each time the solver needs Tridagon, the puzzle's difficulty rating grows by 400. Its class is Extreme. It is needed in fewer than 1 in 5,500,000 puzzles that sudokUI generates. How the rating works.