Swordfish Sudoku Technique

Updated 11 August 2026

A Swordfish is the three-line version of an X-Wing: a digit confined to the same three columns across three rows can be erased from every other cell in those three columns.

When a Swordfish applies

A Swordfish relaxes the X-Wing’s requirement in one useful way. An X-Wing needs exactly two candidate cells in each of two rows. A Swordfish needs three rows whose candidate cells for a digit, taken together, span exactly three columns — but no single row has to use all three columns itself. A row can have the digit in only two of the three columns, as long as the union across all three rows is exactly three columns and no row spills into a fourth.

That flexibility is also why Swordfish patterns are harder to spot than X-Wings. Instead of comparing two neat pairs of cells, a solver has to track a digit across three rows at once and confirm the column count stays at three. Once it does, the logic is identical to the X-Wing: one of the three rows supplies the digit to each of the three columns, so no other row can contribute it there.

As with the X-Wing, the pattern also runs on the column axis: three columns whose candidate cells for a digit span exactly three rows trap the digit just as tightly, eliminating it from those three rows elsewhere.

Worked example

In row 5, 8 can only go in r5c2 and r5c8. In row 6, it can only go in r6c2 and r6c9. In row 9, it can only go in r9c8 and r9c9. Collect the columns those six cells use: 2, 8, 2, 9, 8 and 9 — exactly three distinct columns, 2, 8 and 9, spread across exactly three rows. That is a Swordfish on 8.

Because rows 5, 6 and 9 between them must place 8 in columns 2, 8 and 9, no other row can hold an 8 in any of those three columns. r4c2 was carrying 4, 8 and 9 and loses the 8, leaving 4 and 9. r4c9 was carrying 5 and 8 and loses the 8, leaving 5 alone — which hands the next pass a naked single. No other cell in columns 2, 8 or 9 outside rows 5, 6 and 9 still had 8 as a candidate on this board, so those are the pattern’s only two eliminations.

How to spot a Swordfish

1. Pick a digit and list its candidate cells per rowFor one digit, write down which columns still hold it in each row. Rows where the digit has four or more candidate cells are not useful.
2. Look for three rows spanning three columnsFind three rows whose candidate columns for the digit, combined, total exactly three distinct columns. Each row can use two or three of them.
3. Reject a fourth columnIf any of the three rows has a candidate in a column outside the shared set of three, the pattern does not hold. Every candidate cell in the three rows must sit in one of the three columns.
4. Erase the digit from the rest of the three columnsRemove the digit from every other cell in the three columns, outside the three base rows, then repeat the search on the column axis.

Common mistakes

• Requiring every row to use all three columns. Two candidate columns out of three is enough for a row, as long as the combined span across all three rows stays at three.
• Missing a fourth column hiding in one of the three rows. A single stray candidate cell outside the shared columns invalidates the whole pattern.
• Erasing the digit from inside the three base rows. The eliminations apply to the three columns outside those rows, not to the rows themselves.
• Stopping the search after finding one Swordfish. A board can carry more than one fish on different digits at the same time, and later passes may need a fresh scan.