Swordfish Technique is even rarely used. The following examples are found after hundreds of tries. This technique is an extension of X-Wing Technique since 3 columns and 3 rows instead of 2 columns and 2 rows are involved. Let's first look at an example:

Pay attention to the locations of value 9s in Columns 1, 4 and 5. In Column 1, candidate 9 appears at cells [A1] and [E1]; in Column 4, 9 occurs at [E4] and [I4]; while in Column 5, 9 is found at [A5] and [I5]. That is, value 9 totally has no more than 3 locations in these 3 columns and these locations occupy three rows in total, which are Rows A, E and I.

Now let's list all the possibilities to assign value 9s to these columns:

• Suppose if [A1] = 9, then [A5] <> 9, so for Column 5, 9 can only be assigned to [I5]. In consequence, [I4] cannot be 9, so for Column 4, it's only possible that [E4] = 9.
• Suppose if [E1] = 9, then [E4] <> 9, so for Column 4, 9 can be assigned only to [I4]. Therefore, [I5] must not be 9, so for column 5, [A5] = 9.

In simple words, there are only two cases: value 9 can either be assigned to [A1], [E4] and [I5], or be assigned to [A5], [E1] and [I4]. But whichever case happens, value 9 exists in Rows A, E and I. So candidate 9s in other cells of these three rows should be removed. In this example, 9s at [A6] and [E2] are eliminated.

So here is the prerequisite to apply this technique:

• If a number has only three locations in total in three different columns and these locations happen to lie in the same three rows in total, then this number can be removed from the candidates of the other cells of these three rows.
• If a number has only three locations in total in three different rows and these locations happen to lie in the same three columns in total, then this number can be removed from the candidates of the other cells of these three columns.

An example of Case 2 is given below:

In this example, the locations of value 6 in Rows C, F and H are only in Columns 5, 7 and 8. Therefore, conditions are satisfied for using Swordfish Technique. We can remove candidate 6 from [G7] and [I7] in Column 7, and from [G8] in Column 8.

You can never find Swordfish in blocks.

See Also:

• What is Sudoku: rules, history and terminology
• Sudoku Strategies
• Direct Elimination Techniques
• Candidates Elimination Techniques
• --> Naked Single Technique
• --> Hidden Single Technique
• --> Intersection Removal Technique
• --> Naked Pair Technique
• --> Naked Triplet Technique
• --> Naked Quad Technique
• --> Hidden Pair Technique
• --> Hidden Triplet Technique
• --> Hidden Quad Technique
• --> X-Wing Technique
• --> XY-Wing Technique
• --> XYZ-Wing Technique
• --> WXYZ-Wing Technique