A digit confined to 2-3 cells in each of three rows (or columns), all within three columns (or rows).
Choose a digit and find three rows where it has only two or three possible cells, with all of those cells confined to the same three columns. Each row must use one of the three columns, which accounts for them completely. Eliminate the digit from every other cell in those columns.
A Swordfish is the natural extension of the X-Wing pattern from two rows and two columns to three rows and three columns. Pick a digit: if it appears as a candidate in at most three cells in each of three rows, and all those cells fall within the same three columns, the digit is locked into those nine intersections. It can be eliminated from all other cells in those three columns.
Unlike an X-Wing where each row has exactly two candidate cells, a Swordfish allows rows to have two or three cells. The key requirement is that the union of all column positions across the three rows uses at most three columns.
Swordfish is the second member of the "fish" family. It's rarer than X-Wing but follows the same logic. Understanding X-Wing makes Swordfish straightforward, it's the same reasoning applied to a larger pattern.
Scan for a digit and find three rows where that digit appears in 2-3 cells each. Check whether all candidate cells fall within three columns. If so, the digit is locked: it must appear once in each of those three columns, using these three rows.
Apply the elimination: remove the digit from all other cells in those three columns that are not in the three defining rows.
The same logic works transposed: three columns with 2-3 candidate cells each, all within three rows, eliminating from other cells in those rows.
Step 1: Scanning digit 3, we find three rows with limited positions. In row 3, digit 3 appears in R3C4 and R3C6. In row 6, it appears in R6C1, R6C4, and R6C6. In row 7, it appears in R7C1 and R7C6. All positions fall within columns 1, 4, and 6.
Step 2: This is a Swordfish. Digit 3 must occupy exactly one cell from each defining row within columns 1, 4, and 6. Therefore, 3 can be eliminated from all other cells in columns 1, 4, and 6.
Step 3: Eliminate 3 from R5C1, R8C1, R8C4, and R8C6. The Swordfish locks digit 3 into the three defining rows.
No. A row may have two or three candidate cells for the digit — that is the main difference from an X-Wing, where each row has exactly two. What matters is that the candidate cells across all three rows use no more than three columns in total. Rows contributing only two cells are perfectly valid.
Pick a digit and write down, row by row, which columns can still take it. Ignore any row with four or more positions. Among the rows left, look for three whose column lists together cover only three columns. Then repeat the exercise column by column to catch the column-based version.
Every cell in the three defining columns that does not lie in one of the three defining rows. The pattern's own intersections are untouched, and the defining rows offer nothing to remove because their only candidates for the digit are already inside the pattern. Row-based Swordfish eliminates in columns; column-based eliminates in rows.
Because the argument needs each of the three rows only to place the digit somewhere inside the three columns. Three rows must supply three copies of the digit, and three columns can host exactly three between them, so the columns are fully used. Whether a row offers two or three positions makes no difference to that count.
Size. A Swordfish uses three rows and three columns, with two or three candidate cells per row; a Jellyfish uses four rows and four columns, with two to four cells per row. The reasoning is identical, but Jellyfish is far rarer, and most puzzles that appear to need one yield to simpler techniques first.
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