Peak Sudoku

Finned X-Wing

An X-Wing plus one or two extra candidates (the fin) inside one box.
All techniques

Overview

Tier 4
Fish, wings, chains & uniqueness
Typical difficulty
Peak Score 65–80
Also known as
Finned Fish, Imperfect X-Wing

A Finned X-Wing is an X-Wing with one or two extra candidate cells (the fin) in the same 3×3 box as one of its corners. Eliminations are restricted to cells that see both the fin and the standard elimination cell.

In practical play, nearly complete X-Wings occur frequently, spoiled only by an extra candidate or two in one row. If those extra candidates (the fin) reside in the same 3×3 box as one corner of the X-Wing, we can still deduce a powerful elimination. Either the fin is false (in which case a standard X-Wing exists), or the fin is true (in which case the digit is in that box). Any cell that sees both the standard X-Wing elimination and the fin will be cleared in both scenarios.

Worked examples

1 / 5
Reading the exampleCells forming the patternCells losing candidatesEliminated candidates
In column 8, the digit 7 can only go in rows 5 and 7. Column 6 has the same two plus a fin at R4C6. If the fin is 7, it sees every cell of row 5 in the center box; if not, columns 6 and 8 form an X-Wing on rows 5 and 7. Either way, 7 can be removed from row 5 in the center box.
Removes: 7 from R5C4

How to apply it

Spotting the pattern and making the deduction, in order.

  1. Look for an X-Wing pattern on a digit where one corner has one or two extra candidate cells (the fin) in the same 3×3 box.
  2. Consider outcome 1: If the fin turns out to be false, a clean X-Wing forms and clears the intersecting column.
  3. Consider outcome 2: If the fin turns out to be true, the digit lands in that 3×3 box and clears the rest of that box.
  4. Identify the overlap: cells that lie in both the target column and that same 3×3 box.
  5. Eliminate the candidate from those intersection cells, which are cleared under both outcomes.

Common mistakes

  • Trying to eliminate candidates along the entire column. In a finned fish, eliminations only occur within the box containing the fin.
  • The fin must be in the same 3×3 box as a corner of the fish; if it is outside, the technique does not hold.

Frequently asked questions

Where do eliminations occur in a Finned X-Wing?
Only in cells that see both the fin and the X-Wing corner cell in the same box and column/row.
Peak Score 55–70X-WingA digit restricted to exactly two cells in two different rows, forming a rectangle.Peak Score 60–75SkyscraperTwo rows (or columns) where a digit has exactly two positions, sharing one column (or row).

Put Finned X-Wing into practice

Try a puzzle from the Tough tier (Peak Score 61–70).

Play a Tough puzzleAll 25 techniques