13
Techniques · Hard
W-Wing
Two cells that do not see each other both hold the same two candidates, say A/B. Somewhere else on the board, a row, column or box has exactly two places for A, and one of those places sees the first cell while the other sees the second. If both cells were A, neither place in that unit could take A, which is impossible. So at least one of the two cells is B, and B can be removed from any cell that sees both of them.
How to spot it
Look for two matching two-note cells, such as 4/9 and 4/9, that are far apart. Pick one of the two digits and search for a unit where it has only two places, each lined up with one of the cells. Remove the other digit from the cells where the two can both see.
Example
5
4
7
39
16
2
389
689
9
8
126
5
23
16
4
7
36
23
7
26
8
239
4
1369
139
5
8
19
5
4
6
7
139
2
39
24
24
3
1
5
9
8
6
7
7
6
19
3
8
2
5
19
4
1
239
29
6
4
8
7
5
23
26
5
8
29
7
3
69
4
1
2346
234
7
29
1
5
369
389
23689
r1c5 and r4c9 both hold only 3/9, and 3 has exactly two places in row 2 (r2c5 and r2c9), one seeing each of them. So they cannot both be 3, one of them is 9, and 9 can be removed from cells that see both (W-Wing).
the pattern cells that lose a candidate (crossed out)