Fish

Finned X-Wing

A finned X-Wing is an X-Wing that almost works. One row holds a digit in exactly two cells; a second row holds it in the same two columns, plus one or two extra cells in the same box as one of its corners. Those extra cells are the fin. Either the fin holds the digit, or the second row is a clean X-Wing row and the pattern works as usual, so any cell in a cover column that can also see the fin can't hold the digit.

Written by Georgi Nedyalkov

Level
Advanced

The pattern

Finned X-Wing: what it does.

Without the fin, the four corners would be a plain X-Wing and both columns would be cleared. The fin spoils that, but only partly: it can't hold the digit and leave the X-Wing intact at the same time.

So there are two cases. If the fin doesn't hold the digit, the X-Wing stands and clears both columns. If it does, the digit is ruled out of everything the fin can see. A cell that loses in both cases loses for certain: one that sits in a cover column and in the fin's box.

That usually leaves one or two cells, in the box where the fin and its corner meet. The same pattern read in columns rather than rows works the same way, and finned swordfish and jellyfish follow the same rule with more lines.

Example

Finned X-Wing: a worked example.

  1. Look for an X-Wing that fails

    Follow one digit and find two rows that would make an X-Wing if one of them didn't have an extra candidate or two. The other row has to be clean: exactly two homes.

  2. Check where the extras sit

    The extra candidates in the second row are the fin. They must all be in one box, and that box must also hold one of the row's two corners. Extras spread over two boxes are not a fin.

  3. Find the cells that see the fin

    The cover columns are the two the clean row's homes sit in. The cells of those columns, outside the two rows, that share the fin's box are the only ones in reach.

  4. Remove the digit there

    Clear the digit from those cells. Nothing else in the cover columns can be touched, because the fin might be the digit.

Spotting it

Finned X-Wing: how to spot it.

  • Near misses are the clue. When an X-Wing scan turns up a row with three homes instead of two, look at where the third one sits before moving on.
  • The fin and its corner share a box. If the extra candidate is in a box with no corner, the pattern isn't finned and gives nothing.
  • Expect a small result. A finned X-Wing usually clears one cell, sometimes two, all inside the fin's box.
  • If the corner beside the fin has no candidate at all, the pattern is a sashimi X-Wing. The rule is the same, cells in a cover column that see the fin, but it's easier to misread.

Where it stops working

A finned X-Wing removes less than a plain one, and the removal depends on reading the fin's box correctly: a cell in a cover column outside that box isn't touched. It also needs complete candidates in both rows. One stale mark can make a fin out of nothing, or hide the one that's there.

Keep going

Related topics.

New to sudoku? The rules take two minutes to read, and the first-puzzle walkthrough covers the basics that come before any of these.

Beyond the lessons.

The game's lessons stop at X-Wing, so there's no Finned X-Wing lesson. The eight it does teach will get you through almost any puzzle; this page is here for when you run into the pattern anyway.

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