Swordfish

The Swordfish is an extension of the X-Wing to three rows and three columns. When a single digit appears as a candidate in exactly two or three cells across three different rows, and all those cells collectively span only three columns, the digit can be eliminated from all other cells in those three columns.

The name comes from the irregular, multi-pronged shape the pattern often makes on the grid - Like a swordfish silhouette. It is one of the hardest techniques to spot by eye.

1. Choose a digit and fill full candidate notes.
2. Find all rows where the digit appears in 2 or 3 cells (not more).
3. Select three of those rows. List the columns occupied by the digit's candidates.
4. If the union of columns across all three rows is exactly 3 columns, you have a Swordfish.
5. Eliminate the digit from all other cells in those 3 columns that are not in the Swordfish rows.
6. Check for newly created singles or pairs.