Once you have X-Wing under your belt, swordfish is a short step away. It is the same idea stretched from two lines to three: instead of one digit trapped in two rows and two columns, it is trapped in three rows and three columns. The board still looks stuck. Singles have run dry, pairs have done what they can, and even a scan for X-Wings turns up nothing. Swordfish catches what those miss.
The pattern shows up less often than X-Wing because it needs three rows to line up rather than two, but the payoff can be bigger: a single swordfish sometimes clears candidates from a dozen cells at once. It belongs to a family solvers call "fish" patterns, named for the way the marked candidate cells look like scales when you sketch them on a grid. X-Wing is a two-line fish, swordfish is a three-line fish, and jellyfish (four lines) is the next size up, though jellyfish is rare enough that most solvers never need it by hand.
Do not let the name intimidate you. The logic is identical to X-Wing, just with one more row and one more column to track.
Worked example
A real Expert-tier puzzle, reduced to a stuck position after singles. Step through to see the swordfish on digit 6.
Spotting a swordfish
Choose a digit and look at every row for how many cells still carry it as a candidate. You are looking for rows where the candidate count is two or three, never more. Collect three such rows and check the columns their candidates occupy. If those columns, combined across all three rows, add up to exactly three distinct columns, not four or five, you have found a swordfish.
The tricky part is that not every row needs all three columns. One row might have candidates in columns 2 and 5; another in columns 2, 5 and 9; a third in columns 5 and 9. Between them, the candidates never leave columns 2, 5 and 9, even though no single row touches all three. That is still a valid swordfish. Beginners often expect every row to look identical, the way X-Wing rows do, and give up on a swordfish that is actually there because the rows do not match exactly. They only need to combine to the same three columns.
As with X-Wing, check both directions. A row-based swordfish removes candidates from columns; a column-based swordfish removes candidates from rows. Scan for both before concluding the board has nothing left to offer.
Why three rows and three columns is enough
The proof generalises the X-Wing argument directly. Suppose digit 4 in rows 1, 4 and 8 is confined to columns 2, 5 and 9 (with each row using two or three of those columns). Across the three rows, the digit needs a placement in each of the three rows, and every one of those placements has to land in column 2, 5 or 9, because nowhere else is a candidate for that digit in those rows.
Now think about it from the column side. Column 2 needs its own 4 somewhere in the grid. The only rows among our three that can supply it are rows 1, 4 and 8, since that is where the candidates live. The same is true for columns 5 and 9. With three rows and three columns, and every row's candidate confined to those columns, the three placements exactly use up the three columns between them, one placement per column, distributed across the three rows. Which row supplies which column is not yet fixed, but between them the three columns are already spoken for. So no cell outside those three rows, sitting in column 2, 5 or 9, can hold a 4. That is the elimination, and it holds regardless of which of the several valid arrangements the three rows end up taking.
Common mistakes
- Requiring every row to use all three columns. A swordfish only needs the candidates to combine to three columns total. Individual rows can use two of the three, as long as none strays outside the set.
- Including a row with four or more candidate columns. That row cannot be part of the pattern. Drop it from consideration, even if it seems close.
- Mixing up which lines lose candidates. The three rows (or columns) that define the pattern keep every one of their candidates. The elimination happens in the three columns (or rows) they point to, and only outside the defining rows themselves.
- Assuming swordfish is rarer than it is because it looks complicated. The search is mechanical, just wider than X-Wing. Do not skip checking for it on a stuck Expert board just because the pattern spans an extra line.
Practicing swordfish
This pattern shows up almost exclusively on the hardest boards. On this site that means Expert puzzles, built with 23 to 27 givens and a solver that checks for exactly one solution before the puzzle is ever shown to you. Turn on Notes so the candidates are visible, and use a hint if you get stuck on whether a pattern you found is genuine; you get three hints per puzzle, and Challenge mode adds a timer if you want the practice under pressure. If a board looks stuck and swordfish is not there, double check for an XY-Wing, which catches a different class of stuck position entirely.
Expert boards run 23 to 27 givens and are generated fresh with a guaranteed unique solution. Turn on Notes and look for three rows sharing three columns.
Frequently asked questions
A swordfish is a pattern where one digit's candidates, across three rows, are confined to the same three columns (with two or three candidate cells per row). The digit can then be eliminated from every other cell in those three columns. The row and column roles can also swap.
X-Wing uses two rows and two columns. Swordfish uses three of each. The underlying logic, that the digit's placements in the defining lines must exactly fill the linking lines, is the same argument extended by one line in each direction.
No. Each row just needs its candidates to fall inside the set of three columns; individual rows can use only two of the three. What matters is that, combined across all three rows, the candidates never leave those three columns.
It shows up less often than X-Wing because the setup needs three lines to align rather than two. On Expert puzzles it appears often enough to be worth checking for whenever the board stalls after singles, pairs and X-Wing.
That would be a jellyfish, a related but distinct pattern with four lines and four linking columns. It exists but comes up rarely enough that most solvers handle it, if at all, with a solver tool rather than by hand.
Mainly Expert, with 23 to 27 givens. Hard puzzles occasionally require it. Easy and Medium boards are generated to be solvable with singles alone, so you will not encounter it there.