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
Box Hidden Single
In a box, if only one cell can contain the digit 7 (due to 7s in intersecting rows/columns), then 7 must go there.
Row Hidden Single
If the digit 3 can only go in one cell of a row, it's a hidden single, even if that cell has other candidates.
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
