Finned X-Wing

A Finned X-Wing is an almost-X-Wing pattern with one or more extra candidates — the "fin." While a regular X-Wing allows eliminations along entire rows or columns, a Finned X-Wing restricts eliminations to cells that also see the fin.

What is a Fin?

Consider a regular X-Wing:

        Col A       Col B
Row 1:    X           X
Row 3:    X           X

Four corners, perfect rectangle. Now add a "fin":

        Col A       Col B       Col C
Row 1:    X           X
Row 3:    X           X           X  ← fin

The extra candidate in R3C3 is the fin. It breaks the perfect X-Wing pattern.

Why It Still Works (Partially)

In a regular X-Wing, we eliminate from ALL of columns A and B (except the four corners).

With a fin, we can't eliminate everywhere — but we can still eliminate from cells that:

1. Would be eliminated by the X-Wing, AND
2. Also see the fin

The logic:

Case 1: The fin is true (R3C3 = X)

• Then row 3's X is in column C
• The X-Wing cells in row 3 (A and B) are false
• So row 1 must have X in either column A or B

Case 2: The fin is false (R3C3 ≠ X)

• Row 3 has X in columns A or B only (like a regular X-Wing)
• The X-Wing logic applies normally

The insight:

• If the fin is true, we know where the digit is in row 3 (column C)
• If the fin is false, regular X-Wing logic applies

Either way, cells that BOTH:

• Would be eliminated by the X-Wing
• AND see the fin

...must not have the candidate.

Finding Elimination Targets

For a Finned X-Wing, valid eliminations are in the intersection of:

• The X-Wing elimination zone (the columns in our example)
• The fin's vision (cells sharing row, column, or box with the fin)

Typical targets:

• Cells in the same box as the fin
• That are also in one of the X-Wing columns

Step-by-Step Example

Digit 4:

        Col 2       Col 7       Col 8
Row 1:    4           4                   ← two candidates (good)
Row 6:    4           4           4       ← three candidates (fin at C8)

Pattern analysis:

• Row 1: 4 in columns 2 and 7 (exactly 2 — strong link)
• Row 6: 4 in columns 2, 7, and 8 (three candidates)
• Column 8 candidate is the fin

X-Wing columns: 2 and 7

Fin location: R6C8

Elimination zone:

• Normal X-Wing would eliminate from columns 2 and 7
• With fin, only eliminate from cells that also see R6C8

What sees R6C8?

• Row 6: already part of the pattern
• Column 8: no overlap with columns 2 or 7
• Box 6 (rows 4-6, columns 7-9): cells in columns 7 within this box

Target cells:

• R4C7: in column 7 (X-Wing) and box 6 (sees fin)
• R5C7: in column 7 (X-Wing) and box 6 (sees fin)

Eliminate 4 from R4C7 and R5C7 if present.

Finned X-Wing Positions

The fin can be in different spots:

Fin in Same Box as Corner

        Col A       Col B
Row 1:    X           X
Row 3:    X   fin     X

Fin is in the same box as R3CA. Eliminations are in that box, in column B (and A if it extends into the box).

Fin Outside X-Wing Columns

        Col A       Col B       Col C
Row 1:    X           X
Row 3:    X           X           fin

Fin is outside the X-Wing structure. Eliminations where columns A/B intersect the fin's box.

Multiple Fins

        Col A       Col B       Col C
Row 1:    X           X           fin
Row 3:    X           X           fin

Both rows have fins. The elimination zone shrinks to where both fins' boxes overlap the X-Wing columns.

Finned X-Wing vs. Regular X-Wing

Trade-off: Fins make the pattern more common but reduce elimination power.

Sashimi X-Wing

A Sashimi X-Wing is a special finned variant where:

• One of the X-Wing corners is missing
• The fin "replaces" it in the same box

        Col A       Col B
Row 1:    X           X
Row 3:    X           ·   fin

The corner R3CB is empty, but there's a fin in the same box. The logic still produces eliminations where the box and columns overlap.

Why it works:

• The fin could be true (covering for the missing corner)
• Or the pattern works as a degraded X-Wing

How to Find Finned X-Wings

Method 1: Near-Miss X-Wings

1. Look for X-Wing patterns that almost work
2. Check if the "extra" candidates form a fin
3. Verify the fin is in a box that overlaps the X-Wing columns/rows

Method 2: Strong Link Extension

1. Find a row/column with exactly 2 candidates (strong link)
2. Find another row/column with 2-3 candidates
3. Check if the third candidate forms a usable fin

Method 3: Box Inspection

1. Find a box where a candidate forms an L or line
2. Check if that line aligns with X-Wing potential in other rows/columns
3. The non-aligned cells might be fins

Practice Exercise

Find the Finned X-Wing and elimination:

Digit 2:

• Row 2: columns 1, 4
• Row 7: columns 1, 4, 5 (fin at column 5)
• Box 7 (rows 7-9, columns 1-3) contains R7C1

Common Mistakes

Mistake 1: Treating as regular X-Wing

Finned X-Wings have restricted elimination zones. Don't eliminate from the entire columns/rows.

Mistake 2: Wrong fin identification

The fin is the EXTRA candidate that breaks the rectangle. Make sure you've correctly identified the X-Wing corners.

Mistake 3: Missing the box intersection

Eliminations must see the fin. Usually this means being in the same box. Check carefully.

Mistake 4: Ignoring Sashimi variants

A missing corner doesn't invalidate the pattern if a fin compensates.

Quick Reference

Finned X-Wing definition:

• Almost an X-Wing, plus extra candidate(s) — the fin
• Fin is typically in the same box as an X-Wing corner

Elimination rule:

• Must be in the X-Wing elimination zone (the columns or rows)
• Must ALSO see the fin (same row, column, or box)
• Only the intersection of these zones is valid

Finding it:

1. Look for near-X-Wing patterns
2. Identify the fin
3. Find cells in X-Wing columns/rows that share a box with the fin

Relationship to X-Wing:

• More common than pure X-Wing
• Fewer eliminations per find
• Same underlying logic with additional constraint

When to look:

• X-Wing search finds "almost" patterns
• Digits with 5-6 remaining placements
• Hard and above difficulty