Sudoku Tutorial
Finned Swordfish
This is almost a swordfish. If you look at the three yellow columns,  If the blue cell didn't contain a 1, then candidate 1 would be confined to just 3 rows in those three columns.

But this isn't the case.  The pesky blue cell ruins our swordfish.  So the blue cell is the fin.     Our target cell is now r9c4 (with the slashed 1) because it resides in the  box with the fin  (box 8).   So consider just two possibilites:
  1. The blue cell is a 1.  This would eliminate the slashed 1 in r9c4
  2. The blue cell is NOT a 1.  Now we would have a nice valid swordfish which would also remove the slashed 1
So either way, the slashed 1 can be removed.

(Why are there yellow cells that don't contain a 1?  click here)

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