The swordfish in sudoku
The swordfish scales the X-wing’s rectangle up to a three by three frame. One digit, three rows, and every home the digit keeps in those rows falling inside the same three columns. The pattern takes longer to find than its smaller sibling and pays out more eliminations when it lands. This guide shows it on a real position, explains why it works, and gives you a scan that keeps the search short.
The pattern
Hunt rows where one candidate has two or three homes left. Take the 4s below. The top marked row holds its 4 in two cells. So does the middle one, and so does the bottom one. Six cells between three rows, and together they touch three columns. That agreement is the whole pattern: three rows, one candidate, three shared columns.
Check the claim yourself; the board plays fair. Each marked row keeps one spare empty cell besides its pair, and all three spares stand in the same column as the lone placed 4 near the top right. One placed digit pins three rows at once. Filled cells cover everything else, so the homes you see are the homes there are.
Why the elimination works
Each marked row must put its 4 somewhere, and its options all sit in the three shared columns. No column can host two of them, since a column repeats no digit. Three rows into three columns, one each, whichever way the details fall. Every column in the frame ends up owning a 4 from one of the marked rows, so a 4 penciled anywhere else in those columns has nowhere left to be true:
Three eliminations here, spread across three boxes. A swordfish tends to pay better than an X-wing for a simple reason: three columns shed candidates instead of two, and the knock-on effects reach more of the board.
How to scan for one
Pick a digit with five or six placed copies and select any one of them. The board lights up its remaining candidates, notes included, and Auto notes fills the picture in one tap if your marks are thin. A complete candidate picture matters here even more than with the X-wing, because a missed mark in the wrong cell fakes a qualifying row.
Count homes per row. Two or three qualifies, four or more means skip the row. Note the columns each qualifying row uses, and watch for three rows whose columns fold into one set of three. Most digits fail the test inside seconds, which is fine. The scan costs little, and the digit that fits pays for all the ones that fail.
A row with two homes counts. The textbook diagrams show nine candidate cells in a full grid, but 2-2-2 and 2-3-2 arrangements appear far more often in play, and the board above is a 2-2-2.
Near misses
The trap that costs the most time: three rows that qualify on their own and refuse to agree on columns.
This is the first board with the placed 4 taken away. Each spare cell regains a 4, every row now carries a third home, and a count of the columns comes back at four. Each row passes alone. The union fails, and no elimination exists. Audit a suspected swordfish column-first: all the homes, in all three rows, inside the same three columns, or nothing.
Columns work too
A fish swims both ways. Three columns whose 4s fold into three shared rows pin the digit the same way, and the eliminations run along those rows instead. Column fish hide from habitual row-readers, so when a stalled puzzle refuses to give up a row swordfish, run the identical count down the columns before moving to the next digit.
Frequently asked questions
What is a swordfish in sudoku?
A swordfish is one candidate digit confined, across three rows, to the same three columns. Each of those columns must take the digit from one of the three rows, so the columns are spent, and you can erase that candidate from every other cell in all three. Swap rows and columns and the pattern works the same way.
Does a swordfish need nine candidate cells?
No. The full three-by-three grid of homes is the textbook picture, but any mix works as long as each row keeps two or three homes and all of them fall inside the shared columns. The board on this page is a 2-2-2 swordfish: six cells, same logic, same eliminations.
Why is it called a swordfish?
It belongs to the family solvers call fish, patterns that pin one digit inside a grid of rows and columns. The X-wing is the two-row fish, the swordfish is the three-row version, and the four-row jellyfish continues the theme. The names come from solver tradition, and the fish stuck.
Do expert sudoku puzzles require a swordfish?
Our expert tier is graded to need at least one technique from its rung: a swordfish, an XY-wing, a skyscraper or coloring. A given puzzle might lean on a different one of those, so expect the swordfish to appear often across the tier without appearing in every single puzzle.