Swordfish: Sudoku Technique Explained

Swordfish: sudoku technique

An X-Wing stretched over three rows and three columns.

How it works

Find a number whose possible places in each of three rows all fall within the same three columns. Each row may have two or three spots, as long as no spot falls outside those columns.

Those three rows will place their three copies of the number in those three columns, one per column. So every other cell in those columns can lose that candidate. The pattern works with rows and columns swapped.

Worked example

This example comes from a real puzzle in our collection, at the exact moment the technique applies.

Here is the board at that moment, with the candidates still open in each empty cell. Earlier steps may already have ruled some out.
8
2
67
1
9
3
467
5
467
9
3
1
4
67
5
267
26
8
4
5
67
67
8
2
9
1
3
5
78
4
678
2
1
678
3
9
3
78
2
9
4
678
15
68
15
6
1
9
5
3
78
2478
248
247
2
4
3
68
1
9
568
7
56
1
9
5
2
67
678
3
468
46
7
6
8
3
5
4
12
9
12
In three lines, the number has its only spots inside the same three crossing lines. The cells that matter are highlighted.
8
2
67
1
9
3
467
5
467
9
3
1
4
67
5
267
26
8
4
5
67
67
8
2
9
1
3
5
78
4
678
2
1
678
3
9
3
78
2
9
4
678
15
68
15
6
1
9
5
3
78
2478
248
247
2
4
3
68
1
9
568
7
56
1
9
5
2
67
678
3
468
46
7
6
8
3
5
4
12
9
12
The result: 2 candidates go. 6 leaves row 2, column 7; 6 leaves row 8, column 9.
8
2
67
1
9
3
467
5
467
9
3
1
4
67
5
267
26
8
4
5
67
67
8
2
9
1
3
5
78
4
678
2
1
678
3
9
3
78
2
9
4
678
15
68
15
6
1
9
5
3
78
2478
248
247
2
4
3
68
1
9
568
7
56
1
9
5
2
67
678
3
468
46
7
6
8
3
5
4
12
9
12

Where you'll need it

A Swordfish is the hardest technique you'll need on GridPorch. Nothing in our collection goes beyond it.

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