What is a Swordfish in sudoku?

A Swordfish is an advanced fish pattern for a single candidate digit that spans three rows and three columns. You pick three rows (the base sets) in which the candidate appears in only cells lying within the same three columns (the cover sets). Because the digit must be placed once in each base row and can only go in those three columns, you can eliminate that candidate from all other cells in those three cover columns.

What's the difference between an X-Wing and a Swordfish?

Both are single-digit fish, but an X-Wing uses two base sets and two cover sets (a 2x2 pattern), while a Swordfish uses three base sets and three cover sets (a 3x3 pattern). An X-Wing needs the candidate in exactly two cells per base line, whereas a Swordfish is more flexible: each base row or column may contain two or three of the candidate, as long as the whole pattern stays within three cover lines. In short, a Swordfish is the next size up from an X-Wing.

How do you spot a Swordfish?

Focus on one candidate digit and scan the rows to find three rows where that digit is confined to two or three cells, and where all those cells fall within the same three columns. If the columns line up across the three rows, you have a row-based Swordfish and can eliminate the candidate from those three columns everywhere else. The same search works with the roles of rows and columns swapped for a column-based Swordfish.

Does a Swordfish require exactly three candidates in each row?

No. Each of the three base rows may contain either two or three of the candidate, and the rows do not all have to match. The only strict requirement is that every occurrence across the three base rows lies within the same three cover columns; the pattern still works even when some rows have only two candidates.

Swordfish: Sudoku Technique with Examples

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.

Digit 2 is locked to 3 rows → erase 2 from the crossing columns.

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.

Swordfish