Sudoku Tutorial

Swordfish

Before tackling the swordfish, it is imperative that the fully understand the X-wing The best way to think of a swordfish is like this:  an X-wing is 2 by 2 while a swordfish is 3 by 3.   In an X-wing, a candidate appears twice in two rows and two columns. In a swordfish, a candidate appears three times* in three rows and three columns. In the figures below, the candidate in question is in the yellow rows (twice for the X-wing, three times* for the swordfish).  The red squares indicate the candidate location in the rows.  The blue columns contain the cells where the candidate can be eliminated. In the X-wing, there only two possibilities. They  must be diagonally opposed to each other (like this) X-wing (2 by 2) Swordfish (3 by 3) The swordfish has six possible arrangements of the candidate in question (compared to two possibilities in the X-wing).   The red cells in the figures below indicate the six possible locations of the candidate in the solved puzzle. In each of the six possible arrangements, the candidate is located once in each row and once in each column.  It cannot exist in any of the other cells of the blue columns and can removed from those blue cells. Got it?   *  Well, there's bad news.  There's a problem with swordfish.  (continue)

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