A hidden single is when a digit has only one possible location in a house.
Understanding the Concept
Also called "Unique Candidate" or "Pinned Digit".
Within a row, column, or box, a digit can only go in one cell.
It's "hidden" because the cell might have other candidates.
This is different from a naked single where the cell has only one candidate.
Hidden singles are the most common solving technique.
Every digit 1-9 must appear once in each house, so if only one cell can hold it, it must go there.
Examples
Hidden Single in Box 0
In the top-left box, where can 4 go? R0C2 already has candidates {1,2,4,6} but 4 can't go in any other empty cell of this box (they see 4 elsewhere). So 4 must go at R0C2 (green) - it's hidden among other candidates.
5
3
123456789
7
6
123456789
123456789
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
Solution
Pro Tips
- •Check each digit 1-9 in each house
- •Look where each number CAN'T go to find where it MUST go
- •Hidden singles are more common than naked singles
- •Master this technique - it solves most easy/medium puzzles
