Sudoku Tutorial
A jellyfish is a 4 by 4 version of a swordfish   (swordfish are 3 by 3)
This jellyfish is defined by the rows, eliminations occur in the colums.  
jellyfish.jpg A Jellyfish is present when a candidate is confined to four columns in four selected rows.

The problem with jellyfish is that the candidate in question (5 in this case) can appear 2, 3, or 4 times in the four designated rows.  This makes them particulary hard to spot.  I've highlighted all 16 cells here to show the 4 by 4 nature of a jellyfish.  But as you can see, a 5 is not present in all of the yellow cells.

(confused?  click here for further explanation)

The good news is you'll probably never need to find a jellyfish. There's great X-wing of 5's here (look in rows 1 and 9). There's also a swordfish of 5's in rows 1,6, and 9.

But now you can say that you've seen a jellyfish.

A 5 by 5 version of this is known as a Squirmbag.  Its another technique that you'll never need.  But click the link if you want to see one.

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