Sudoku Fish Techniques: A Roadmap From X-Wing to Jellyfish

What Sudoku fish techniques actually are

Every fish looks at just one candidate at a time, say the blue hexagon that Shapedoku draws for the digit two. You ignore all other shapes and study only where that one candidate can live. A fish forms when the candidate is restricted to a small number of rows, and inside those rows it only appears in the same small set of columns. That symmetry is the whole trick. Because each of those rows must place the candidate somewhere, and the only columns available are shared, those columns get fully claimed. Any copy of the candidate sitting in those columns but outside the chosen rows can be eliminated. Fish are named by size: two lines make an X-Wing, three make a Swordfish, four make a Jellyfish. The logic never changes, only the number of lines you track at once, which is exactly why Sudoku fish techniques reward a patient, ordered approach.

Begin with the X-Wing

The X-Wing is the gateway fish and the one you should drill until it is automatic. Picture the candidate two restricted to exactly two columns in r2 and r6. If in both rows the only spots are c3 and c8, you have four corners: r2c3, r2c8, r6c3, r6c8. Whichever way the two settles, it fills one corner in each row and forces the diagonal partner, so columns c3 and c8 will each hold a two inside these rows. That means every other two in c3 and c8, such as one sitting at r4c3, can be crossed out. Practise spotting X-Wings by scanning a single candidate row by row and asking where it is pinned to just two cells. The Shapedoku Notes view, with its smart cleanup, makes these twin pairs easy to see because stray marks are trimmed automatically.

Grow into Swordfish and Jellyfish

A Swordfish is the same idea across three rows and three columns, but with a gentle relaxation: each row does not need all three columns, it just may not use any column outside the set. So r1 might use c2 and c5, r4 might use c5 and c9, and r7 might use c2 and c9. Together they still lock columns c2, c5 and c9, and any two in those columns outside r1, r4 and r7 falls. A Jellyfish extends this to four lines. Jellyfish are genuinely rare because by the time a digit is confined to four columns, a simpler move has usually appeared first. Learn to recognise a Jellyfish so you are not blind to it, but do not go hunting for one before the smaller fish are second nature.

Finned, sashimi, and the exotic fish

Real puzzles are messier than diagrams, and that is where finned fish arrive. A finned fish is an almost-perfect fish with one or two extra candidates, the fin, spoiling the clean pattern. The eliminations still work, but only for cells that see both the normal fish and the fin. A sashimi fish is a finned fish so damaged that one corner is missing entirely, propped up by the fin alone. These sound advanced, yet they appear far more often than textbook fish, so they repay study. Franken and mutant fish widen the definition further by mixing rows, columns and boxes as the base and cover sets. They are the deep end, and most solvers never need them for anything below the hardest Extreme puzzles. Because Shapedoku separates shape from colour, Sudoku fish techniques are easier to spot here than in a numbers grid, since one glowing candidate stands out at a glance. When a fish defeats you, the free solver at shapedoku.com can reveal one so you can study its silhouette and then find the next yourself.

• X-Wing: master first, scan one candidate for two-cell rows or columns.
• Swordfish: three lines, columns need not all be used, only shared.
• Jellyfish: four lines, rare, learn to recognise rather than to hunt.
• Finned and sashimi: messy real-world fish, very common, high value.
• Franken and mutant: box-based fish, save for the very hardest puzzles.