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Swordfish

When three lines lock a candidate into the same three crossings.

Explanation

A Swordfish is the three-line version of X-Wing. If a candidate in three rows can only appear inside the same three columns, remove that candidate from the other cells in those columns. The same rule works with rows and columns swapped.

Swordfish example Sudoku board 3 4 7 2 7 1 9 8 1 8 3 5 6 7 8 4 2 8 1 9 2 4 8 6 5 4 2 8 7 9 6 5 4 2 7 9
An example board where Swordfish applies

Practice tip

Scan one digit at a time after easier candidate eliminations. Swordfish patterns are rare but useful in very hard puzzles.

Example steps

  1. Pick one candidate and find rows with two or three positions.
  2. Find three rows whose positions use only three columns.
  3. Remove that candidate from the other cells in the crossing columns.

Try this technique on a real puzzle.

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