TECHNIQUE · EXPERT
Swordfish — X-Wing extended to 3×3
If X-Wing is a 2-row 2-column cross pattern, Swordfish is its 3×3 generalization. When a digit's candidates in three rows are confined to the same three columns, those columns can be cleared in every other row.
The logic
Digit N appears only within columns {X, Y, Z} across rows A, B, and C. Each row places N once, consuming one of the three columns. So columns X, Y, Z cannot hold N in any other row.
Relation to X-Wing
X-Wing is the 2×2 special case of Swordfish. Extending to 4×4 gives Jellyfish. The underlying principle — n rows confine a digit to n columns — scales to any size.
How to find it
Use the same process as X-Wing, but for three rows. Map candidate columns per row and look for a 3-row combination whose column union is exactly three.
- Pick an unplaced digit.
- Record candidate columns per row (each row with 2–3 candidates).
- Find a 3-row group whose column union has exactly 3 elements.
- Remove the digit from those 3 columns in all other rows.
Practice order
- Pick a digitChoose a digit with many remaining candidate cells.
- Map per rowRecord which columns hold the digit in each row.
- Find the 3-row groupLook for three rows whose column union has exactly 3 elements.
- Eliminate columnsRemove the digit from those 3 columns in all other rows.
Walk through a Swordfish
Step 1 of 4
Focus on digit 3. In rows 1, 5, and 9, the only candidate cells for 3 land within columns 2, 5, and 8.