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Swordfish & Jellyfish
Master the Swordfish: a three-line fish where one digit confined to three rows across three columns is eliminated from the rest of those columns. Jellyfish too.
The Swordfish is the X-Wing grown by one line. Take a single digit and find three rows where it appears in only the cells of three shared columns. Between them, those three rows must place the digit into those three columns — so it can be removed from every other cell of the columns. As always, rows and columns are interchangeable.
Unlike the X-Wing, the rows need not each hold the digit exactly twice — two or three positions are fine, as long as the three rows together span only three columns. That flexibility is what makes a Swordfish trickier to spot.
A Jellyfish is the next size up: four rows confined to four columns. The same logic scales, but eliminations of this size are rare in practice. The example highlights the three-by-three Swordfish frame and the candidates it clears.
Practise the Swordfish
The best way to learn a technique is to use it. Play a puzzle at the level where it first appears, or drop a tricky board into the solver to watch it in action.
Frequently asked questions
What is a Swordfish in Sudoku?
A single-digit pattern across three rows whose candidates for that digit fall in just three columns. The digit is then removed from those three columns everywhere outside the pattern (or rows and columns swapped).
How is a Swordfish different from an X-Wing?
An X-Wing uses two rows and two columns; a Swordfish uses three of each. The rows in a Swordfish can have two or three candidate positions, not just two.
What is a Jellyfish?
The four-line version of the same fish logic: four rows confined to four columns. It is valid but uncommon, since a simpler technique usually resolves the board first.
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