Swordfish Sudoku Strategy Explained

Worked example: a Swordfish on 6

Step 1 of 9

Find the three base rows

Take a look at this puzzle. 6 is the fish digit, and rows 1, 6, and 9 are the base sets.

Sudoku Swordfish example with three base rows for digit 6

The candidates align in three columns

The candidates for 6 in rows 1, 6, and 9 line up perfectly in three columns: columns 1, 2, and 9.

Swordfish candidates aligned in three columns

One way the 6s can fall

There are only a few ways for 6 to sit in those three rows. One possible arrangement is shown here.

One possible arrangement of 6s inside the Swordfish

And another way

Here is another possible arrangement. Whichever way the 6s fall, each base row places its 6 in one of the three aligned columns.

Another possible arrangement of 6s inside the Swordfish

The base sets cover the columns

Either way, the three base rows supply the 6 for each of the three aligned columns. So 6 cannot appear anywhere else in those columns.

Swordfish base rows covering three columns

Eliminate in column 1

For instance, in column 1 the digit 6 must sit at R6C1 or R9C1, so 6 can be eliminated from all other cells in column 1.

Eliminate in all three columns

To conclude: 6 can be removed from every other cell in the three covered columns.

Candidate 6 eliminated from all Swordfish columns

A Swordfish on 7

Here 7 is the fish digit and rows 2, 3, and 7 are the base sets. Each base row has three or fewer candidates, and together they stay within three columns.

Swordfish on digit 7 with base rows 2, 3, and 7

Eliminate along the columns

The candidates for 7 align in columns 2, 5, and 9, so 7 can be eliminated from the other cells in those columns.

Candidate 7 eliminated from three columns by Swordfish

Swordfish vs X-Wing

Swordfish works exactly like X-Wing, but with three base sets instead of two. When a digit is restricted to at most three cells in each of three rows, and all those cells fall in the same three columns, the digit can be eliminated from every other cell in those columns. The same works with columns as base sets.

If you have not learned X-Wing yet, start there: Swordfish is the same fish logic on a bigger frame.

Sudoku X-Wing StrategyX-Wing is a fish pattern where one candidate is restricted to the same two columns in two rows, or the same two rows in two columns.

Worked example: a Swordfish on 6

In this puzzle the fish digit is 6 and the base sets are rows 1, 6, and 9.

Walk through this in steps 1-7 of the guided example above

Second example: base rows with fewer candidates

Not every base set needs three candidate cells. Each base row may have two or three possible cells, as long as all of them fall inside the same three columns.

Walk through this in steps 8-9 of the guided example above

How to search for a Swordfish

Swordfish is rare in easy and medium puzzles. Search for it in evil puzzles when singles, locked candidates, pairs, and X-Wing have run out.

Pick one digit and mark or highlight all its remaining candidates.
Look for three rows where the digit has two or three possible cells each.
Check whether all those cells fall inside the same three columns.
If they do, remove the digit from every other cell in those three columns.
Repeat with columns as base sets, eliminating along rows.

Sudoku Pencil Marks and CandidatesPencil marks, also called candidates, are small notes that show which digits may still fit in an empty cell.Sudoku X-Wing StrategyX-Wing is a fish pattern where one candidate is restricted to the same two columns in two rows, or the same two rows in two columns.

Frequently asked questions

What is the difference between Swordfish and X-Wing?

X-Wing uses two base rows or columns; Swordfish uses three. Both restrict a single digit and eliminate it from the crossing lines.

Do all base rows need three candidates?

No. Each base row may have two or three candidate cells, as long as every candidate falls inside the same three columns.

How common is Swordfish?

It is an advanced pattern that mostly appears in the hardest puzzles. Check simpler techniques first; a Swordfish search is worth it when everything else is stuck.