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Jellyfish

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

Explanation

A Jellyfish extends Swordfish to four lines. If a candidate in four rows can only appear inside the same four columns, remove that candidate from the other cells in those columns. The same rule also works with rows and columns swapped.

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

Practice tip

Use this after easier candidate eliminations. Jellyfish is uncommon, so scan one candidate at a time and look for four aligned lines.

Example steps

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

Try this technique on a real puzzle.

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