Swordfish
A Swordfish is an X-Wing with three rows and three columns. If a digit has at most three candidate cells in each of three rows, and all of those cells fall in the same three columns, then the three rows use up the digit in those columns, and the digit can be removed from every other cell of the three columns. Rows and columns can be swapped. Bare Sudoku checks for Swordfish right after X-Wing, in Expert puzzles.
When to use it
When an X-Wing finds nothing. Swordfish are less common than X-Wings and need a digit whose candidates are spread thinly over three lines. They need complete and accurate notes for that digit.
How to spot it
Pick a digit and look for rows with two or three candidate cells for it. If three such rows cover only three columns in total, check those columns for further candidates of the digit. Not every row needs a candidate in every column: a Swordfish can have six, seven, eight or nine cells. In the game, highlight the digit with its key and look for three rows whose candidates line up in three columns.
Example
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | |
|---|---|---|---|---|---|---|---|---|---|
| r1 | 3 | 14 | 7 | 5 | 6 | 19 | 8 | 2 | |
| r2 | 1489 | 6 | 489 | 48 | 3 | 2 | 1479 | 17 | 5 |
| r3 | 2 | 5 | 48 | 7 | 19 | 6 | 3 | 14 | |
| r4 | 149 | 8 | 49 | 3 | 5 | 6 | 14 | 2 | 7 |
| r5 | 6 | 5 | 2 | 1 | 4 | 7 | 3 | 9 | 8 |
| r6 | 7 | 14 | 3 | 2 | 9 | 8 | 5 | 6 | 14 |
| r7 | 48 | 9 | 48 | 6 | 2 | 5 | 17 | 17 | 3 |
| r8 | 5 | 7 | 1 | 9 | 8 | 3 | 2 | 4 | 6 |
| r9 | 2 | 3 | 6 | 7 | 1 | 4 | 8 | 5 | 9 |
Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.
- Digit 1 has at most three candidate cells in each of rows 2, 4, 7, and all of those cells fall in columns 1, 7, 8: r2c1, r2c7, r2c8, r4c1, r4c7, r7c7, r7c8.
- Each of the three rows must place its 1 in one of the three columns, and no two rows can use the same column. So the three columns get their 1 from these three rows.
- Any other 1 candidate in columns 1, 7, 8 is impossible. Remove 1 from r1c7, r3c1.
Common mistakes
- Requiring all nine cells. A Swordfish with two candidates in some rows is still valid, as long as the three rows cover only three columns.
- Including a row with four candidate cells. Each base row must have at most three, all within the three columns.
- Eliminating in the base rows. With rows as the base, the eliminations are in the three columns, outside the base rows.
In Bare Sudoku
Needed from level: Expert
In the game, the hint says: Swordfish
Play this puzzle · Play Sudoku · Sudoku solver
Practice puzzles
In these puzzles, Swordfish is the hardest technique you need. Each link opens the puzzle in the game; the solver link shows every step.
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
- Expert 3.8 · Sudoku solver
Frequently asked questions
What is a Swordfish in Sudoku?
A digit whose candidates in three rows are confined to three columns. The three rows must place the digit in those three columns, one each, so the digit is removed from the other cells of those columns.
How is a Swordfish different from an X-Wing?
Only in size: an X-Wing uses two rows and two columns, a Swordfish three and three. The reasoning is the same.
Does a Swordfish need exactly nine cells?
No. Each row can have two or three candidates; the pattern only requires that the three rows together use no more than three columns.
Is there a fish of size four?
Yes, the Jellyfish, with four rows and four columns. The reasoning is the same as for the X-Wing and the Swordfish, only with more lines.
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