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Easy

Hidden Single

A Hidden Single flips the logic of a Naked Single. Instead of asking “what can this cell be?”, it asks “where can this digit go?” within a row, column, or box — even if the cell that turns out to hold it still shows other candidates too.

Why it works

If a digit is only possible in one cell within a unit — a row, column, or box — that's where it has to go, regardless of what else that cell could theoretically hold. The candidate is “hidden” because the cell itself doesn't look forced until you check the whole unit.

Worked example

5
3
7
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 1 of 3

Pick the unit

Look at Column 8. Where can 1 go?

5
3
7
6
1
9
5
×
9
8
6
8
6
×
3
4
8
3
×
1
7
2
×
6
6
2
8
4
1
9
×
5
8
7
9
Step 2 of 3

Rule out cells

1 cannot go in any of the crossed-out cells, because each already sees a 1. That leaves one place.

5
3
7
1
6
1
9
5
9
8
6
8
6
3
4
8
3
1
7
2
6
6
2
8
4
1
9
5
8
7
9
Step 3 of 3

Place it

1 can only go in R1C8 within Column 8, so put it there.

Tips

KaizenDoku spots every one of these patterns automatically after each game, and shows you exactly where you used them on your own board.

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More techniques

Naked SingleNaked PairHidden PairPointing PairsBox Line ReductionX-WingSwordfishUnique Rectangle