Swordfish Technique in Sudoku: Three-Line Fish Guide

What is a Swordfish?

A Swordfish is an X-Wing stretched across three lines. It is the next member of the “Fish” family.

When a candidate digit appears in two or three cells in each of three columns, and all of those cells sit in the same three rows:

1. Each of the three columns must place that digit once, so the three columns place three copies of it.
2. All three copies must land in the same three rows, one per row.
3. So each of those three rows gets its copy from inside the three columns.
4. Therefore, that digit cannot appear anywhere else in those three rows! (Swap rows and columns and the same logic works the other way.)

Worked Example: Swordfish on Digit 4 Eliminates 4 from Cell R5C2

Follow candidate 4 down Column 3, Column 7 and Column 9:

• Column 3: 4 can only go in R1C3, R2C3 or R5C3.
• Column 7: 4 can only go in R2C7 or R5C7.
• Column 9: 4 can only go in R1C9 or R2C9.
• The Three-Row Lock: Every candidate sits in Row 1, 2 or 5. Each column needs one 4, and no row can hold two, so the three 4s fill Rows 1, 2 and 5 exactly once each, all inside Columns 3, 7 and 9. No two columns form an X-Wing on their own, so all three are needed.
• Target Elimination at R5C2: Cell Row 5, Column 2 (R5C2) has candidates {1, 4}. Row 5's 4 is already claimed by the Swordfish, so candidate 4 is eliminated from R5C2, leaving 1 as a Naked Single!
• Bonus eliminations: R1C1 and R1C2 in Row 1 also lose their 4.