The Finned X-Wing Technique in Sudoku
What Is a Finned X-Wing?
A Finned X-Wing is an almost-complete X-Wing with one or more extra candidate positions in one base unit. These extra positions are the fin. The fin must share a block with one corner of the underlying X-Wing, and eliminations are limited to cells in that block that also lie on the relevant cover unit.
How to Identify a Finned X-Wing
- Choose one candidate digit.
- Find two base rows with two aligned cover columns.
- Confirm that one base row has an extra candidate outside those cover columns.
- Check that the fin shares a block with an X-Wing corner.
- Eliminate from the relevant cover column inside the fin block.
Candidate 4 appears at R2C3 and R2C8, and at R5C3, R5C8, and fin R5C9. If the fin is false, the remaining cells form a standard X-Wing. If the fin is true, it directly sees cells in block 6. In both cases, candidate 4 can be removed from R4C8 and R6C8.
Why the Elimination Is Local
Outside the fin block, the extra candidate prevents a standard X-Wing conclusion. Only cells that see the fin and lie on the affected cover column are eliminated in both logical branches.
Finned X-Wing and X-Wing
A standard X-Wing has no candidates outside its two cover units. A finned pattern permits extras, but gains fewer, local eliminations.
What Are Common Finned X-Wing Mistakes?
Do not eliminate along the entire cover column. The target must be in the fin's block, on the correct cover unit, and outside the base row.
Checking Both Logical Branches
If R5C9 is not 4, the four body cells form a standard X-Wing and remove other 4s from C8. If R5C9 is 4, it removes 4 from its block directly. R4C8 and R6C8 are the overlap of those conclusions; cells in other blocks are not.
Frequently Asked Questions
Can a Finned X-Wing have several fins?
Yes, provided the fins belong to the same relevant block and support the same overlap elimination.
Why can the whole cover column not be cleared?
When the fin is true, it controls only its block. Cells outside that block are not eliminated in both branches.
Summary
Treat the pattern as an X-Wing body plus a same-block fin, then keep only the overlap between the fish elimination and the fin's block.