Hidden single: sudoku technique

The only place left for a number in its row, column or box.

How it works

Instead of asking what can go in a cell, pick a number and ask where it can go. Take a box that is missing a 7. If the 7s elsewhere on the board block every empty cell in that box but one, the 7 goes there.

The cell may still have several candidates, which is why this single is hidden. Scanning one number at a time across the board is the quickest way to find them.

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
68
2
5
3
68
4
9
1
5
4
9
1
2
7
6
3
8
1
3
68
4689
48
689
7
5
2
3
28
7
28
5
1
9
4
6
68
268
1
7
9
4
5
28
3
4
9
5
3
6
28
128
28
7
2
1
468
468
48
568
38
7
9
9
5
3
268
7
268
28
1
4
68
7
468
24689
148
3
28
268
5
Inside this group, only one cell can still hold the number. The cells that matter are highlighted.
7
68
2
5
3
68
4
9
1
5
4
9
1
2
7
6
3
8
1
3
68
4689
48
689
7
5
2
3
28
7
28
5
1
9
4
6
68
268
1
7
9
4
5
28
3
4
9
5
3
6
28
128
28
7
2
1
468
468
48
568
38
7
9
9
5
3
268
7
268
28
1
4
68
7
468
24689
148
3
28
268
5
The result: row 9, column 5 must be 1.
7
68
2
5
3
68
4
9
1
5
4
9
1
2
7
6
3
8
1
3
68
4689
48
689
7
5
2
3
28
7
28
5
1
9
4
6
68
268
1
7
9
4
5
28
3
4
9
5
3
6
28
128
28
7
2
1
468
468
48
568
38
7
9
9
5
3
268
7
268
28
1
4
68
7
468
24689
1
3
28
268
5

Where you'll need it

Hidden singles are the workhorse of easy and medium boards, and they turn up in the later stages of every harder puzzle too.

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Practice on an easy sudoku