Hidden pair: sudoku technique

Two numbers that can only go in the same two cells.

How it works

Look at a group and find two numbers that each appear as candidates in only two cells, and the same two cells at that. Those cells must hold those two numbers, whatever else is pencilled in them.

So every other candidate in those two cells can go. Once you erase them, the pair is out in the open, and it often sets off further moves.

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.
7
4
2
3
9
6
5
8
1
8
1
3
2
5
4
6
7
9
5
6
9
1
78
78
2
3
4
3
2
56
7
4
158
89
1569
68
1
57
567
9
28
3
4
56
268
4
9
8
6
12
125
37
15
237
29
57
157
4
1237
127
3789
69
3678
29
3
17
8
6
127
79
4
5
6
8
4
5
37
9
1
2
37
Two numbers appear as candidates only in these two cells of the group. The cells that matter are highlighted.
7
4
2
3
9
6
5
8
1
8
1
3
2
5
4
6
7
9
5
6
9
1
78
78
2
3
4
3
2
56
7
4
158
89
1569
68
1
57
567
9
28
3
4
56
268
4
9
8
6
12
125
37
15
237
29
57
157
4
1237
127
3789
69
3678
29
3
17
8
6
127
79
4
5
6
8
4
5
37
9
1
2
37
The result: one candidate goes. 6 leaves row 5, column 9.
7
4
2
3
9
6
5
8
1
8
1
3
2
5
4
6
7
9
5
6
9
1
78
78
2
3
4
3
2
56
7
4
158
89
1569
68
1
57
567
9
28
3
4
56
268
4
9
8
6
12
125
37
15
237
29
57
157
4
1237
127
3789
69
3678
29
3
17
8
6
127
79
4
5
6
8
4
5
37
9
1
2
37

Where you'll need it

Hidden pairs are one of the patterns that separate hard boards from medium ones. They're harder to spot because the extra candidates disguise them.

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Practice on a hard sudoku