Techniques › Swordfish
Swordfish
An X-Wing with three rows and three columns instead of two.
The same argument as the X-Wing, one size up. Three rows in which a digit has two or three possible cells, all falling within the same three columns. The digit must occupy one cell in each row, and therefore one in each column — so it clears from the rest of those columns.
Importantly the rows do not each need all three columns. A Swordfish where the rows use columns {1,4}, {4,7} and {1,7} is completely valid, and is the shape that gets missed.
A worked example
Taken from a real puzzle in our bank, at the point where this was the cheapest move available. Pattern candidates are marked in amber; anything the step eliminates is struck through in red.
| 7 | 5 | 36 | 4 | 36 | 8 | 1 | 2 | 9 |
| 9 | 8 | 2 | 5 | 36 | 1 | 4 | 7 | 36 |
| 4 | 36 | 1 | 7 | 2 | 9 | 8 | 36 | 5 |
| 2 | 4 | 9 | 1 | 5 | 6 | 3 | 8 | 7 |
| 3 | 7 | 5 | 9 | 8 | 2 | 6 | 4 | 1 |
| 6 | 1 | 8 | 3 | 7 | 4 | 9 | 5 | 2 |
| 1 | 9 | 367 | 2 | 4 | 37 | 5 | 36 | 8 |
| 8 | 2 | 34 | 6 | 1 | 5 | 7 | 9 | 34 |
| 5 | 36 | 3467 | 8 | 9 | 37 | 2 | 1 | 346 |
- NudgeDigit 3 lines up in a rectangular pattern across several lines. Nothing outside it can be 3.
- WhereSwordfish in row 1, row 2, row 8, column 3, column 5, column 9: r1c3, r1c5, r2c5, r2c9, r8c3, r8c9
- Do itRemove r7c3 ≠ 3, r9c3 ≠ 3, r9c9 ≠ 3
How to spot it
Mark the rows where a digit has two or three homes. Look for three whose column positions union to exactly three columns. Then transpose and repeat.
Where you will meet it
Swordfish sits at tier 4 of our ladder, which means it first becomes necessary at Expert. Puzzles below that band never require it, though it may still appear as a shortcut.