Sudoku technique
The Swordfish
An X-Wing stretched over three lines instead of two. The logic is no harder; seeing it is, because a real Swordfish almost never looks like a tidy pattern on the board.
What a Swordfish is
A Swordfish is an X-Wing with one more line. Three rows in which a digit’s candidates all fall inside the same three columns lets you remove that digit from those three columns everywhere else — and, as ever, it works with rows and columns swapped.
The reasoning is identical. Three rows each need the digit once. Between them they have only three columns to put it in. Three demands, three slots: the columns are fully committed, and every other cell in them loses the digit.
The part that catches people out
An X-Wing needs exactly two candidates in each row. A Swordfish does not need exactly three in each — it needs two or three, as long as they all live inside the same three columns.
That is why Swordfish are so much harder to see than X-Wings. You are not looking for a neat 3×3 arrangement of nine cells. You are looking for a ragged set of seven, eight or nine cells whose columns happen to collapse to three. Most real Swordfish look like nothing at all until you list the columns out.
Step 1 of 6
A real position
Part way through a hard grid. Every simpler move has been played out, and the 4s are what is left to reason about.
How to look for one
Work one digit at a time and write down, for each row, the columns where that digit is still possible. Ignore rows with four or more — they cannot be part of a three-column pattern. Then look for any three rows whose column sets union to exactly three.
Written as sets it is quick. {2,5}, {2,7} and {5,7} union to {2,5,7} — a Swordfish, and one that looks like nothing on the grid. Doing this on paper beats staring at the board.
Is it worth learning?
For expert grids, yes. Below that, rarely — most puzzles that look like they need a Swordfish actually have a hidden single or a naked pair sitting unnoticed, and it is worth a second sweep for those before committing to the column bookkeeping.
Learn it for the boards that genuinely need it, and for the satisfaction — there are few better moments in Sudoku than spotting one and watching a stuck grid come apart.
Other techniques
Try it on a real board
The free board on the home page has a notes mode, which is what every technique past hidden singles needs. Then take the same eye into a live match.