Swordfish
The Swordfish is the big brother of the XWing. It is more complex and of course more difficult. The Swordfish is considered a difficult higher method.
Just like the XWing, the Swordfish can have two perspectives: as seen from the row or from the column. The Swordfish is a special case in the family of multidimensional groups.
If you look at it from the row perspective, the rule is:
A Swordfish occurs if you have 3 rows where the same candidate appears exactly twice, and the possible fields for this candidate are in exactly 3 columns. In this case the candidate can be deleted from all fields in the columns that do not belong to the 3 rows.
For the column perspective, the rule is:
A Swordfish occurs if you have 3 columns where the same candidate appears exactly twice, and the possible fields for this candidate are in exactly 3 rows. In this case the candidate can be deleted from all fields in the rows that do not belong to the 3 columns.
Look carefully at the picture below. Read it row by row. You will notice that there are exactly 3 rows in which the 1 is a candidate in exactly two fields: rows 1, 4, and 9.
Now consider these fields. In the 3 rows, taken together, the 1 can be a candidate in exactly 6 fields. These 6 fields are in exactly 3 columns: 4, 7, and 9.
These 6 fields for the 1, marked in pink and green, form the Swordfish for the candidate 1.
Just like the XWing, this design can also lead to only two possible solutions. Either all pink fields or all green fields will, in the end, contain the 1. Any combination of green and pink is impossible.
It is therefore logical that the 1 can no longer be a candidate in the yellow fields. In each of these fields it would torpedo both the green and the pink solution.



see also: Generalization: Multidimensional groups
