Swordfish
A Swordfish is a size-3 basic fish for one digit. In one orientation, the digit’s candidates in 3 base rows are confined to 3 cover columns; the transposed orientation swaps rows and columns. The digit can be removed from the cover sectors outside the base sectors.
What is a Swordfish?
The size counts 3 required placements, 3 bases, and 3 covers—not a requirement for all 9 intersections to contain candidates. Each base contributes at least one candidate, and every relevant base candidate lies in the selected covers.
How to spot it
Choose one digit and select 3 rows whose candidates are confined to the same 3 columns, or 3 columns confined to the same 3 rows. Sparse intersections are valid. Then inspect the cover sectors outside the selected bases.
The logic
Each of the 3 base sectors must place the digit, and all 3 placements lie within the 3 cover sectors. Those placements occupy the covers, so any other occurrence of the digit in a cover but outside the bases can be removed. Size-one pointing or claiming is smaller than an X-Wing.