Finned X-Wing

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.

1. 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.

2. 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.

3. 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.

4. Apply the result

   Erase 1 from r3c3. The pattern may remove candidates without immediately solving a cell.