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Swordfish
Three rows confine a digit to three shared columns.
Swordfish is X-Wing's three-row sibling. Take three rows where a digit's remaining candidates only ever land in some three shared columns — collectively. No individual row needs to use all three columns; it just needs to use only columns from that set.
Because the digit must appear once in each of those rows, the three columns will collectively receive it three times — once per row. So the digit can be removed from those three columns everywhere else.
When the move applies
Swordfish are less common than X-Wings but reliable on tough puzzles. They're easier to spot when you've been keeping digit-by-row candidate sketches.
The procedure
- Pick a digit.
- For each row, list the columns where the digit is still a candidate.
- Find three rows whose lists together cover exactly three columns.
- Eliminate the digit from those three columns in every other row.