Finned X-wing
An X-wing where one of the four corners has an extra candidate cell — a fin — in its row or column. The eliminations restrict to cells that see both the X-wing and the fin.
A finned X-wing is an X-wing pattern that almost fits but for one extra candidate cell — the fin — sitting in the same unit as one of the four X-wing corners. The pure X-wing eliminates the digit from every other cell of the two columns. Adding a fin restricts that elimination: only cells that see both the X-wing's eliminations and the fin lose the digit.
Why the fin restricts the elimination
The X-wing argument runs by saying the digit must occupy two of the four corners on a diagonal. If a fifth candidate cell exists in one of the X-wing's rows or columns — outside the four corners — that fifth cell breaks the strict argument. The digit might be at the fifth cell instead of in the X-wing pattern. So the X-wing's normal eliminations don't all hold.
What does still hold: any cell that would be eliminated by the X-wing and by the fin. The two cases — "the X-wing fires, and the fifth cell isn't the digit" or "the fifth cell is the digit, in which case the X-wing's row or column changes" — both eliminate the same target cell, as long as the target cell sees both the X-wing's normal target and the fin.
The eliminations are narrower than a clean X-wing's, but they still fire. Finned X-wings are the most common fin variant; finned swordfish and finned jellyfish exist by analogy and produce more elimination opportunities since the parent fish has more.
The sashimi sub-case
A sashimi X-wing is a finned X-wing with one corner missing entirely — the fin replaces a corner instead of adding to it. The argument carries through: the missing corner means the X-wing can't fire normally, but the fin's existence means the digit must be at the fin or at the surviving corner of that row, both of which eliminate the same target cells the X-wing would have eliminated.
In practice, finned and sashimi variants are catalogued together because the elimination logic is identical. Only the visual shape differs — finned has five candidate cells, sashimi has three plus a fin.
When you'll see it
Finned X-wings appear regularly on hard and expert puzzles where a clean X-wing was almost there. Spotting them is a matter of disciplined "scan for the pattern, then check whether one extra cell is loose." Once you've made finned variants part of your X-wing scan, they stop feeling like a separate technique and become the natural extension.
See also
- X-wing— When a digit's only two cells across two rows form a rectangle in two columns — eliminating that digit from the rest of those columns. Or the same shape rotated 90°.
- Swordfish— The X-wing's three-row counterpart. When a digit's possible cells across three rows fall in the same three columns, that digit can be eliminated from those columns elsewhere.
- Jellyfish— The four-row, four-column generalisation of swordfish. A digit confined to the same four columns across four rows lets you eliminate it elsewhere in those columns.
- Candidate— A digit (1–9) a cell could still legally hold — one not yet ruled out by anything in its row, column, or 3×3 box. Every empty cell has between one and nine.
Read more
- Why the X-wing keeps tripping people up
The X-wing is a famous Sudoku technique that almost everyone knows and almost no one spots in time. Here's the perceptual problem behind it.