Swordfish
Swordfish works the same way as X-Wing, but with 3 base sets instead of just 2.
Take a look at the following Sudoku puzzle.
6 is the fish digit and rows R1, R6 and R9 are the base sets.
The candidates for digit '6' in R1, R6 and R9 align up perfectly in 3 columns: C1, C2 and C9.
The candidates for digit '6' in R1, R6 and R9 align up perfectly in 3 columns: C1, C2 and C9.
There are two options for 6 to reside in those rows, one option is shown here:
and the other one is this one:
Either way, those 3 base sets cover the aligned columns, meaning that 6 cannot appear anywhere else in those columns.
For instance - in column 1 the digit 6 will either reside in R6C1 or R9C1 and we can therefor eliminate 6 from all cells in column 1:
To conclude - we can eliminate 6 from all other cells in those 3 columns.
Here is another example for a Swordfish.
7 is the fish digit and rows R2, R3 and R7 are the base sets.
Not all base sets must have 3 possible candidates.
Each of the base set row must have 3 or less (no more) candidates, as long as they are aligned into three rows.
The candidates for digit '7' in the base sets align up in the same 3 columns - C2, C5 and C9.
We can therefor we can eliminate 7 from other cells in those rows:
