Jellyfish: four rows reserve four columns

Count units, not sixteen corners

For one digit, find four unfilled source rows whose complete candidate positions fit into four columns. Four required placements occupy different columns, reserving those columns. Remove the digit there outside the source rows. The transposed column-based argument also works.

HoDoKu — Basic Fish · SudokuWiki — Jellyfish

Our original positions for 6

Let row A allow 6 only at A1/A6, row C at C3/C8, row F at F1/F8 and row H at H3/H6. Their combined columns are {1,3,6,8}.

Each row needs one 6, with no repeated column. Targets D1, E3, I6 and I8 therefore lose 6 if it was present. We need not decide the four actual placements first. This is a partial one-digit scenario.

Missing corners are fine; an extra column is not

The pattern does not need all sixteen intersections. Our example uses eight. But an additional candidate at A9 creates five possible columns, breaking the four-rows-reserve-four-columns argument. Do not overlook it just because it lies outside the shape you hoped to find.

Check smaller patterns too

A smaller X-Wing or Swordfish might provide a simpler deduction first. A bigger technique name is not stronger evidence; complete source-unit checking is what makes the elimination sound.

In Classic 9×9, use Scan to scan the current entries, inspect justified eliminations and open this lesson. Chain and ALS searches are bounded; UR Type 1 and BUG+1 require verified uniqueness. Reading this lesson never changes your saved game.

Try before looking

References and further reading

Read the source explanations and compare them with our examples. Links are references, not endorsements or claims of affiliation.

• HoDoKu — Basic Fish
• SudokuWiki — Jellyfish