Learn / Fish with fins
Sudoku technique · Fish with fins · first needed in Expert puzzles
Finned X-Wing
Extra candidates spoil an ordinary fish. A target can still be removed when it is excluded both by the fin case and by the no-fin fish case.
How to spot it
Separate core candidates from fins. Test both possibilities: a fin is true, or every fin is false.
The common mistake
The target must be in a cover line outside the bases and must see EVERY fin. Do not erase the entire cover line.
Learn this first
Basic fish.
A worked example
This position comes from SudokuByte’s lesson on the Finned X-Wing. Gold cells hold the evidence; green cells are the pattern’s ends.
Where to look
Separate core candidates from fins. Test both possibilities: a fin is true, or every fin is false.Extra candidates spoil an ordinary fish. A target can still be removed when it is excluded both by the fin case and by the no-fin fish case.
Read the evidence in this position
For 1, the base houses are row 2, row 8. The covers are column 3, column 8. The extra candidates, called fins, are r2c2.Why the deduction must follow
There are two exhaustive cases. A fin is 1: a target that sees every fin is excluded. No fin is 1: the 2 base lines must put 2 copies of 1 into the 2 cover lines. No cover line has room for another 1 outside the bases. Only targets excluded in both cases can be erased.Apply the result
Erase 1 from r3c3. The pattern may remove candidates without immediately solving a cell.
Further reading: hodoku.sourceforge.net