Swordfish

Use when a single candidate is confined to the same three columns (or rows) across three lines.

1. 1

   Candidate 2 is confined to the same three columns across three rows.

   
2. 2

   The 2 is then removed from those columns in every other row.

   

This method is especially useful for unblocking stuck situations in an advanced game. In a practical case, if you observe that in rows 1, 4 and 7, number 5 can only go in the same three columns, you have identified a Swordfish. Now you can safely remove number 5 from columns 2, 5 and 8 in all other rows, which often clears multiple cells and facilitates the solution of the rest of the sudoku.

• Requiring all three cells in each row — a swordfish works even if a row uses only two of the three columns.
• Mixing the orientation: eliminate from columns when the base sets are rows, and vice versa.