Swordfish
Swordfish · 3×3 fish
The X-Wing extended to three rows and three columns.
Rule
Three rows have d only within the same three columns → remove d from those columns in every other row.
Three rows, three columns
Digit 5 in rows 1, 6 and 9 lies only in the three columns 1, 6 and 9 (each row has 2 or 3 places).
The candidates on this board were already reduced by earlier eliminations in this puzzle's solution.
The idea
When three rows need three d's, and every place for d in those rows falls within three columns, each column gets exactly one d from those three rows. The other cells of the three columns have no room for d.
Each row doesn't need all three places; 2 or 3 per row is fine, as long as together they use only three columns.
Extending to four rows and four columns gives a Jellyfish, which is very rare. X-Wing, Swordfish and Jellyfish are together called fish patterns.
How to spot it
- List the rows with d in 2–3 places, and find three rows whose columns together number only three.
- If rows don't work, try columns.
Common mistakes
- A row with d in four places cannot take part.
- Eliminate only in the three cover columns, not in the three base rows.
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Related techniques
References
- HoDoKuSwordfish(opens in a new tab)
- HoDoKuJellyfish(opens in a new tab)
- SudokuWikiSwordfish Strategy(opens in a new tab)
The examples on this page are generated from random puzzles, not taken from these sources.