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Naked single: sudoku technique

A cell where every number but one is already taken.

How it works

Look at one empty cell and check its row, its column and its box. Each number already in one of those three groups is ruled out for this cell. If eight of the nine numbers are gone, the ninth has to go here.

It is the most direct move in sudoku and the first one to look for. Each number you place removes candidates from the cells around it, so placing one single often creates another.

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.
9
7
4
6
3
1
2
8
5
1
6
8
2
5
4
7
3
9
35
2
35
7
9
8
14
146
146
2
1
9
4
8
5
3
67
67
347
8
367
1
2
36
9
5
467
345
35
356
9
7
36
8
146
2
357
35
1357
5
4
2
6
9
8
6
4
237
8
1
9
5
27
37
8
9
125
3
6
7
14
124
14
One cell is left with a single candidate. The cells that matter are highlighted.
9
7
4
6
3
1
2
8
5
1
6
8
2
5
4
7
3
9
35
2
35
7
9
8
14
146
146
2
1
9
4
8
5
3
67
67
347
8
367
1
2
36
9
5
467
345
35
356
9
7
36
8
146
2
357
35
1357
5
4
2
6
9
8
6
4
237
8
1
9
5
27
37
8
9
125
3
6
7
14
124
14
The result: row 7, column 4 must be 5.
9
7
4
6
3
1
2
8
5
1
6
8
2
5
4
7
3
9
35
2
35
7
9
8
14
146
146
2
1
9
4
8
5
3
67
67
347
8
367
1
2
36
9
5
467
345
35
356
9
7
36
8
146
2
357
35
1357
5
4
2
6
9
8
6
4
237
8
1
9
5
27
37
8
9
125
3
6
7
14
124
14

Where you'll need it

Naked singles appear in every puzzle at every level. On easy boards, they and hidden singles are all you need.

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Practice on an easy sudoku