Tier 3 · first asked for by Tough
Swordfish
The X-Wing pattern with three rows and three columns instead of two.
Also called Fish of size 3.
Take three rows in which a digit has only two or three possible cells. If between them those cells occupy exactly three columns, the digit must fill one cell in each of the three rows, and so one in each of the three columns.
It can therefore be removed from those columns everywhere else. Unlike an X-Wing the pattern is rarely a tidy rectangle — the cells can be scattered, and no row needs to use all three columns.
Our solver rejects any swordfish that contains a smaller fish, so an example here is always genuinely three-row reasoning and not an X-Wing with a spare row attached.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it rules out · 5 a candidate that goes
What it proves. Swordfish on 7: in column 3, column 4, column 9 the digit is confined to row 7, row 8, row 9, so it leaves those rows elsewhere (R7C2, R9C1).
Builds on X-Wing.