Swordfish and Jellyfish are the two large members of the fish family - patterns spanning three or four rows and columns respectively. Both appear in expert and extreme Sudoku, both extend the same elimination logic from X-Wing, and both reward methodical scanning. Knowing how they differ in size, frequency, and search effort helps you decide where to invest your scanning time.
A Swordfish is found when a candidate appears in two or three cells across three rows, and those candidates collectively occupy only three columns. Jellyfish extends this to four rows and four columns. The candidate need not appear in every cell of the pattern - only the row/column alignment matters.
Swordfish appears in many expert puzzles. Jellyfish is significantly rarer and primarily lives in extreme and championship-level Sudoku. If you are scanning a hard puzzle and start considering Jellyfish, step back and re-examine for missed singles, pairs, or simpler fish.
To find Swordfish: pick a digit, list rows that contain exactly two or three candidates, then look for any three rows whose candidate cells fall in the same three columns. Jellyfish search uses the same method but with four rows and four columns. The key is being systematic - random scanning rarely catches these patterns.
Swordfish eliminates the candidate from any non-pattern cells in the three columns. Jellyfish eliminates from non-pattern cells in the four columns. Both patterns also work in the column-orientation: scan columns for the pattern and eliminate from the corresponding rows.
Learn Swordfish first - it is more useful per hour of study. Once Swordfish becomes reliable, Jellyfish becomes a natural extension. Many expert puzzles can be solved with Swordfish alone; very few require Jellyfish to make progress. Always exhaust simpler techniques first.
Size. Swordfish uses three rows and columns; Jellyfish uses four. The elimination logic is identical.
Swordfish, by far. Jellyfish is rare and mostly limited to extreme puzzles.
Usually not. Swordfish covers most cases. Jellyfish is often a luxury technique for extreme puzzles.
No - by definition the patterns require different numbers of rows. However, finding a Swordfish often eliminates candidates that would have formed a larger Jellyfish.
Swordfish takes a few hours of focused practice. Jellyfish takes considerably longer because patterns are noisier and examples are rarer.
Mathematically yes, but in 9×9 Sudoku any fish of size five or larger has an equivalent smaller fish via the complement principle, so they are never needed.
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