The Swordfish Sudoku Technique, Explained

A quick recap of the X Wing

The Swordfish builds directly on the X Wing, so it genuinely helps to have that simpler pattern fresh in your mind before you go any further. 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, forming the four neat corners of a rectangle. That tidy rectangle is what lets you remove the value from the rest of those two columns, because the value is effectively trapped into occupying the corners, one per row, and cannot escape them. The Swordfish is the very same trick at heart, just scaled up from two rows and two columns to three of each, with a little more flexibility in how the individual cells are allowed to be arranged. If the X Wing still feels shaky to you, it is well worth solidifying first, because almost everything that makes the Swordfish work is really the same underlying logic simply stretched across one more row and one more column.

What a Swordfish looks like

A Swordfish uses three rows instead of two, and that is the main thing to hold onto as you learn it. Pick a single value to hunt with, say the triangle in Shapedoku or the number seven in classic Sudoku, and follow only that value for a while. If the value is a candidate in only two or three cells across each of three separate rows, and every one of those candidate cells falls within the same three columns, then you are looking at a Swordfish. The genuinely important part is that the entire pattern stays confined to three rows and three columns between them, with nothing leaking outside that frame. Unlike the stricter X Wing, the value does not have to appear at every single intersection of those rows and columns. It only has to stay inside the three shared columns on each of the three rows, which is what makes the Swordfish both more flexible and a little harder to see than its two-row cousin when you are first learning to recognize it on a real board.

A worked example

Take the triangle and look carefully at where it can go, working through the grid row by row. Suppose that in row 2 it can only sit in c3 or c6, that in row 5 it can only sit in c3 or c9, and that in row 8 it can only sit in c6 or c9. Every one of those candidate cells lives inside columns 3, 6 and 9, and no candidate for the triangle in those rows falls anywhere else. That is a Swordfish. The logic that follows is worth stating plainly, because it is what makes the elimination trustworthy rather than a guess. The triangle must fill exactly one cell in each of those three rows, and since every one of those cells sits in column 3, 6 or 9, the triangle is forced somewhere inside those three columns across the whole pattern. It therefore cannot appear anywhere else in any of those three columns. So you may now safely erase the triangle as a candidate from every other cell in columns 3, 6 and 9, for example from r1c3, from r4c6, and from r7c9. That single confident sweep often unlocks cells that had been completely frozen for the entire puzzle.

How to spot one

1. Choose one single value and look only at that value across the entire grid for a moment.
2. Find the rows where that value has just two or three candidate cells remaining.
3. Check whether three such rows happen to share the same three columns for those cells.
4. If they do, you have a Swordfish, so eliminate the value from every other cell in those three columns.
5. Then repeat the whole search along columns instead of rows, since the pattern works equally well in both directions.

When to reach for it

The Swordfish only really matters on genuinely hard puzzles, the ones where scanning, naked pairs, pointing pairs and even the X Wing have all quietly run dry and left you staring at a frozen board. You will not need it every day, and that is completely fine, because it is a specialist tool rather than an everyday one. But when a board is truly locked and stubbornly refuses to give up a single new placement, patiently tracking one value across three rows is very often the exact move that finally breaks the whole thing open again. Learn it slowly and without pressure, practice spotting it on a few genuinely tough boards rather than rushing, and in time it becomes another reliable key hanging on your ring alongside the simpler techniques you already trust. If you want somewhere calm to practice advanced patterns without any worry of spoiling a real board, the Extreme difficulty in Shapedoku is built for precisely this kind of deep, patient logic, and every Extreme puzzle is guaranteed to hold exactly one clean solution waiting to be found.