Finned X-Wing Sudoku Technique

Updated 11 August 2026

A Finned X-Wing is an X-Wing that has picked up one extra candidate cell, called a fin, in one of its base rows or columns, and still supports a smaller but safe set of eliminations near that fin.

When a Finned X-Wing applies

A plain X-Wing needs exactly two candidate cells in each of its two base rows. Real boards are less tidy than that, and a digit often has a third candidate cell in one of the rows, sitting outside the two shared columns. That extra cell is the fin. A fin does not automatically break the pattern; it just means the digit could be placed there instead of at the fish’s corner in that row, so the clean rectangle logic no longer covers the whole cover line unconditionally.

What still holds is narrower but genuine: any cell that would be eliminated by the ordinary X-Wing, and that also sees the fin cell — shares its row, column or box — cannot hold the digit either way. If the fin turns out to hold the digit, that cell is eliminated because it sees the fin. If the fin does not hold the digit, the row collapses back into a clean X-Wing and the cell is eliminated by the ordinary rectangle logic. Either branch removes the candidate, so the elimination is safe even though the fin’s own value is still unknown.

Worked example

In row 4, the digit 1 is a candidate at r4c5 and r4c7. In row 9, it is a candidate at r9c5 and r9c7 as well — the same two columns — which is exactly an X-Wing shape. Row 9 also carries 1 at r9c8, a third cell outside columns 5 and 7. That third candidate is the fin: it stops the pattern from eliminating 1 across the whole of columns 5 and 7, because row 9 might end up placing its 1 at the fin instead of at r9c5 or r9c7.

r9c8 sits in the bottom-right box together with r7c7, r8c7 and the rest of that 3×3 block. r7c7 was carrying 1, 5 and 6 before the deduction. If the fin at r9c8 holds 1, r7c7 cannot, because they share a box. If the fin does not hold 1, row 9 falls back to the clean X-Wing on columns 5 and 7, and r7c7 cannot hold 1 for that reason instead. Either way 1 is impossible at r7c7, so the solver removes it and leaves 5 and 6 behind.

How to spot a Finned X-Wing

1. Look for an almost-X-WingFind two rows where a digit’s candidates mostly line up in the same two columns, but one row has exactly one extra candidate cell outside those columns.
2. Identify the finThe extra candidate cell is the fin. If a row has two or more extra cells, the pattern does not qualify as a Finned X-Wing.
3. Find cells that see both a cover line and the finLook at the cells the ordinary X-Wing would have cleared, and keep only the ones that also share a row, column or box with the fin.
4. Eliminate the digit from those cells onlyRemove the digit from that narrower set of cells. Cells that see a cover line but not the fin are not provably clear.

Common mistakes

• Eliminating the digit from the whole cover line, as a clean X-Wing would. A finned pattern only clears cells that also see the fin.
• Allowing more than one fin cell per base line. Two or more extra candidates usually mean the fish needs a bigger frame, not a fin.
• Forgetting to check whether the fin sits in the same box as a candidate cell, not just the same row or column. Box visibility is often the only link available.
• Treating the fin as a placement. The fin is still just a candidate; the technique never decides whether it holds the digit, only that either outcome clears the target cells.