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# Solving Techniques 9 Sword-Fish

This is a situation where the 1 is only possible in three places, like columns 2, 5 and 8, below. (there are only three cells where 1 can be, in the columns indicated with red lines, and they occur in the same rows)

In this case, the 1’s can’t be in the places marked with x’s as pictured below, so they can be eliminated as candidates.

When examined, you see that there are only six ways in which the 1 can be entered, and that indeed, they can’t be entered where the x’s are.

As a variation there is a pattern like the one below.

It’s a very efficient technique when this pattern actually happens, since you can eliminate lots of candidates at once.

# Finned Sword-Fish

As in the diagram below, when there are many candidates for 1 in the middle column only, it is called a finned sword fish. The cells indicated with x only, can be eliminated as candidates for 1. (also possible when the pattern is not in the center)

Lets examine if this is true. As shown below, if there is a 1 in the ‘fin’ area, there can’t be another 1, other than in this box. However, in column 2 and column 8, there are three places each for where 1 could be. Therefore, the possibilities can’t be eliminated the same way as they can, when there are no fins.

As shown below, if a part that is not the fin becomes a 1, then a 1 can’t be in 3 rows, just as it would, when there are no fins.

Therefore, the candidates that can be eliminated based on the overlap, are the cells that would get eliminated in a swordfish, but only within the box, which the fin is located in.

The swordfish is quite challenging, but when examined, it does hold up to be true.

## Names of cells in Sudoku

 A1 A2 A3 A4 A5 A6 A7 A8 A9 B1 B2 B3 B4 B5 B6 B7 B8 B9 C1 C2 C3 C4 C5 C6 C7 C8 C9 D1 D2 D3 D4 D5 D6 D7 D8 D9 E1 E2 E3 E4 E5 E6 E7 E8 E9 F1 F2 F3 F4 F5 F6 F7 F8 F9 G1 G2 G3 G4 G5 G6 G7 G8 G9 H1 H2 H3 H4 H5 H6 H7 H8 H9 I1 I2 I3 I4 I5 I6 I7 I8 I9