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Naked pair: sudoku technique

Two cells share the same two candidates, so nothing else in their group can use those numbers.

How it works

Suppose two cells in a row can each only be 3 or 8. You don't yet know which is which, but between them they will use both numbers. That means no other cell in that row can be 3 or 8, and you can erase those candidates.

The same reasoning works in columns and boxes. If the two cells also share a box, you can clear the numbers from both groups.

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.
17
567
4
25
8
12
3
9
26
8
36
123
7
9
123
5
4
26
9
35
23
4
235
6
7
1
8
17
79
19
25
25
8
4
6
3
4
8
5
3
6
7
9
2
1
3
2
6
1
4
9
8
5
7
2
4
8
6
37
5
1
37
9
6
1
39
89
37
4
2
378
5
5
39
7
289
1
23
6
38
4
Two cells in the same group hold exactly the same two candidates. The cells that matter are highlighted.
17
567
4
25
8
12
3
9
26
8
36
123
7
9
123
5
4
26
9
35
23
4
235
6
7
1
8
17
79
19
25
25
8
4
6
3
4
8
5
3
6
7
9
2
1
3
2
6
1
4
9
8
5
7
2
4
8
6
37
5
1
37
9
6
1
39
89
37
4
2
378
5
5
39
7
289
1
23
6
38
4
The result: one candidate goes. 2 leaves row 9, column 4.
17
567
4
25
8
12
3
9
26
8
36
123
7
9
123
5
4
26
9
35
23
4
235
6
7
1
8
17
79
19
25
25
8
4
6
3
4
8
5
3
6
7
9
2
1
3
2
6
1
4
9
8
5
7
2
4
8
6
37
5
1
37
9
6
1
39
89
37
4
2
378
5
5
39
7
289
1
23
6
38
4

Where you'll need it

Naked pairs are one of the two new patterns on medium boards. To see them, you need your pencil marks written in.

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Practice on a medium sudoku