sudokUI › Sudoku techniques › Franken Swordfish
Franken Swordfish in sudoku
Three base units (rows or boxes) share no candidate for X, all their X candidates lying in three cover units (columns or boxes) sharing none, or rows and columns swapped.
Why it works
Each base unit places X once, in separate cells, and each cover unit has room for one X, so all three cover units get theirs from the base. Remove X from every cell of the cover units that lies outside the base units.
How to spot it
Filter on X and test three rows or boxes against three columns or boxes: at least one of the six units must be a box.
Implemented, but never needed: an easier technique always reaches the same result first.
In a puzzle's rating
Each time the solver needs Franken Swordfish, the puzzle's difficulty rating grows by 350. Its class is Unfair. It is needed in fewer than 1 in 5,500,000 puzzles that sudokUI generates. How the rating works.