Swordfish

A Swordfish occurs when in three rows all possible cells for a given digit lie in three columns. This digit can be eliminated from the remaining cells of these columns. The technique also works when we replace rows with columns.

In the following sudoku diagram in the second, fourth and eighth rows, the digit 7 might only be placed in the first, second and sixth columns (candidates highlighted in blue). You can eliminate the digit 7 (marked in red) from the remaining cells of these columns. After applying this method, only the digit 5 can be placed in the R1C6 cell.



In the sudoku diagram below, in the first, fifth and eighth columns, the digit 5 can only be placed in the fifth, eighth and ninth rows (candidates marked in blue). You can eliminate the digit 5 (marked in red) from the remaining cells of these rows. After applying this technique, only the digit 7 can be placed in the R5C6 cell.