The Swordfish Sudoku Technique, Explained

A quick recap of the X Wing

The Swordfish builds on the X Wing, so it helps to remember that one. In an X Wing, a single value appears as a candidate in exactly two cells on each of two rows, and those cells line up in the same two columns. That rectangle lets you remove the value from the rest of those two columns. The Swordfish is the same trick, just larger.

What a Swordfish looks like

A Swordfish uses three rows instead of two. Pick a single value, say the triangle in Shapedoku or the number seven in classic Sudoku. If that value is a candidate in only two or three cells across each of three rows, and all of those cells fall within the same three columns, you have a Swordfish. The key is that the whole pattern is confined to three rows and three columns.

The elimination it gives you

Because the value must go in one cell per row and each of those cells sits in one of the three shared columns, the value is forced to occupy those three columns within those rows. That means it cannot appear anywhere else in those three columns. So you can safely erase the value as a candidate from every other cell in the three columns. That single sweep often frees up cells that were completely locked.

How to spot one

1. Pick one value and look only at that value across the whole grid.
2. Find rows where the value has just two or three candidate cells.
3. Check whether three such rows share the same three columns for those cells.
4. If they do, you have a Swordfish. Eliminate the value from the other cells in those three columns.
5. Then repeat the search along columns instead of rows, since the pattern works both ways.

When to reach for it

Swordfish only matters on genuinely hard puzzles, the ones where naked pairs, pointing pairs and even the X Wing have run dry. It is not something you need every day, but when a board is truly locked, scanning for a single value across three rows is often the move that breaks it open. Learn it slowly, practice spotting it on a few tough boards, and it becomes another reliable key on your ring.

If you want a place to practice advanced patterns without the fear of ruining a real board, the Extreme difficulty in Shapedoku is built for exactly this kind of deep, patient logic.