An X-Wing with one extra candidate cell, the "fin." Eliminations are restricted to cells that also see the fin.
Look for a would-be X-Wing on one digit that is spoiled by a single extra candidate in one of the two rows — the fin. Either the clean X-Wing holds or the fin holds the digit, so eliminate only where both cases agree: cells in the two shared columns that also share the fin’s box.
A Finned X-Wing is what happens when you almost have a perfect X-Wing, but one row (or column) has an extra candidate cell, the "fin." In a regular X-Wing, a digit appears in exactly two cells in each of two rows, and those cells share the same two columns, allowing broad eliminations. The fin breaks this symmetry.
Despite the imperfection, the pattern is still useful. The logic works by considering two possibilities: either the standard X-Wing holds (and eliminates as usual), or the fin cell contains the digit (and eliminates from its own peers). The intersection of these two cases (cells that would be eliminated in both scenarios) can still be safely eliminated.
This makes Finned X-Wing more common than regular X-Wing but with fewer eliminations. You will encounter it frequently in Extreme-level puzzles.
Start by looking for an almost-X-Wing: a digit appears in exactly two cells in one row (the "base"), and in two or three cells in another row. The extra cell in the second row is the fin.
Identify the two columns that would form the X-Wing (the columns shared by the base and the non-fin cells in the cover row). The fin cell is in the same row but a different column.
Now consider: if the fin cell does NOT contain the digit, the remaining cells form a standard X-Wing and you can eliminate from both shared columns. If the fin cell DOES contain the digit, it eliminates the digit from its peers. The cells that lose the digit in BOTH cases are the safe eliminations, specifically, cells in the shared columns that also see the fin cell (share its box).
Example 1: Finned X-Wing
Look at digit 1 in rows 5 and 7. In row 5, digit 1 appears as a candidate in R5C3 and R5C4, a clean pair. In row 7, digit 1 appears in R7C1, R7C3, and R7C4. If only R7C3 and R7C4 had digit 1, we would have a perfect X-Wing in columns 3 and 4. R7C1 is the fin, the extra cell.
Green circles mark digit 1 in the pattern cells. The fin (R7C1) is in box 7. Only cells in columns 3-4 that also share box 7 with the fin can be eliminated. R8C3 and R9C3 meet this criteria, they are in column 3 and in box 7.
Eliminate digit 1 from R8C3 and R9C3. R9C3 reduces to {7}, a Naked Single.
Because the fin might be the cell that holds the digit, in which case the X-Wing never forms and its column-wide eliminations are not justified. Only cells that lose the digit under both possibilities are safe, and those are the cells in the shared columns that also share a box with the fin.
The fin is the extra candidate cell that stops a Finned X-Wing from being a clean X-Wing. One row — the base — has the digit in exactly two cells; the other row has it in three: the two that line up in the same columns, plus one more. That third cell is the fin.
Take the two columns the X-Wing would occupy and keep only the cells that also share a box with the fin. Those cells lose the digit either way: if the fin is empty of it the X-Wing clears the columns, and if the fin holds it the fin clears its own box.
Yes — a Finned X-Wing works in either orientation, exactly as a plain X-Wing does. A row-based one has its base and finned lines in rows and eliminates within the two shared columns; a column-based one has them in columns and eliminates within the two shared rows, again only in cells sharing the fin’s box.
Yes — Finned X-Wings turn up more often than clean X-Wings, so scanning for them pays off. Each one removes fewer candidates than a full X-Wing, but they appear frequently in Extreme puzzles, and the pattern is easy to check once you already know the plain X-Wing shape.
A Finned Swordfish is the same idea one size larger: instead of two rows and two columns it uses three of each, with the digit confined to two or three cells per row, and again one extra cell breaks the pattern. The same two-case logic applies, so eliminations stay restricted to the fin’s box.
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