sudokUI › Sudoku techniques › Turbot Fish
Turbot Fish in sudoku
4-candidate X-Chain · how it fits
Digit X forms two conjugate pairs on four different cells; an end of one pair sees an end of the other, and the remaining ends do not see each other.
Why it works
The two ends that see each other cannot both be X, so at least one of the remaining ends must be X. X is eliminated from every cell outside the pattern that sees both remaining ends.
How to spot it
Skyscraper and 2-String Kite are special cases; otherwise look for shapes where at least one of the conjugate pairs lies in a box.
Worked example
Turbot Fish on 2: r5c7 and r9c7 are the only places for 2 in column 7, as are r7c6 and r9c5 in box 8, and r9c7 sees r9c5. If r5c7 is not 2, r9c7 is, so r9c5 is not, so r7c6 is: one of r5c7 and r7c6 is 2, and 2 is removed from every cell that sees both. So 2 is removed from r5c6.
Practise it on a real puzzle. sudokUI generates a puzzle that needs Turbot Fish and takes you straight to the position where it applies. Hints draw the pattern on the board and explain the step.
In a puzzle's rating
Each time the solver needs Turbot Fish, the puzzle's difficulty rating grows by 120. Its class is Hard. It is needed in 1 in 60 puzzles that sudokUI generates. How the rating works.