How to solve a Swordfish in Sudoku
A Swordfish is a digit confined to the same three columns across three rows, which clears it from the rest of those columns.
How it works
It is an X-Wing with three rows and three columns. If a digit is limited to the same three columns in three different rows, it will fill those columns.
It can be removed from all other cells in those three columns. The same applies with rows and columns swapped.
How to spot it
Choose a digit and mark the rows where it has two or three spots. If three such rows use only three columns between them, you have a Swordfish.
A worked example
This is a step from a real puzzle, a moment where the next move needs a Swordfish.
Struck-through digits are the ones that can be removed.
1 is limited to the same 3 rows across 3 columns (R4C2, R7C2, R2C7, R4C7, R2C8, R7C8). So 1 can't appear elsewhere in those rows: remove it from R2C9.
Where it fits in Rocksled Sudoku
Puzzles that need a Swordfish are never rated below Expert. Each step of this kind adds 70 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.