Swordfish extends X-wing from two rows to three. The shape is irregular and the search is harder, but the eliminations can be huge.

How it works

Pick a digit. Find three rows where the digit has two or three candidate cells. If every candidate cell across those three rows sits inside the same set of exactly three columns, you have a swordfish.

Because each of the three rows must place the digit somewhere in the swordfish, those three placements distribute across the three columns. One per row, one per column. So the digit cannot appear anywhere else in those three columns. Erase it from every cell in the three columns outside the swordfish.

Unlike X-wing, the shape does not need to be a clean rectangle. A row can carry the digit in two of the three columns, or all three. What matters is that no candidate cell leaks outside the column set.

Three rows with digit 5 candidates fall inside three columns. The rest of those columns can be cleared.

When to look for it

After X-wing searches come up empty. Swordfish is rare. If your puzzle is hard enough to need it, candidate sets are tight and every row has only a handful of homes for the digit.

A good signal: a digit appears in seven of the nine rows and the remaining two rows each have three candidate cells. There is a decent chance a swordfish is hiding in those rows plus an already- partial row.

Step-by-step example

Pick a digit, say 5. List the rows where 5 has two or three candidate cells.
Pick any three of those rows and write down the columns that carry candidates for the digit.
If the union of those columns is exactly three, you have a swordfish.
Erase the digit from every cell in those three columns outside the three rows.
Re-scan. Swordfish often produces a single inside the cleared cells.

X-wing, swordfish, jellyfish

	X-wing	Swordfish	Jellyfish
Rows involved	2	3	4
Columns covered	2	3	4
Frequency	Hard puzzles	Expert	Evil and rare
Search cost	Low	Medium	High

The fish family. Each size doubles the search work.

Tips for spotting it

Each row in the swordfish must hold two or three candidate cells. Four or more disqualifies the row.
The columns must be the same set across all three rows. A column that appears in two rows but not the third is not a column of the fish.
Swordfish in columns work the same way. Always scan both directions.
Pencil marks must be clean. A stray candidate breaks the pattern.

Common mistakes

Including a row with four candidate cells for the digit. Swordfish caps at three per row.
Mismatching column sets. All three rows must share the same three columns.
Eliminating from the rows instead of the columns. The cleanup sweeps the columns.
Spending hours on swordfish when an X-wing on the same digit would have done the job.

Practise it

Expert puzzles occasionally require swordfish to break through. Mark candidates fully, then scan one digit at a time across rows. Once swordfish becomes recognisable, the rest of the fish family follows the same pattern. just bigger numbers.