Pick a unit — a row, column or box — and a digit.
Hidden Singles
The most powerful basic technique — a digit with only one home.
A hidden single is a digit that has only one legal cell within a unit — a row, column or box — even though that cell may still list other candidates. The digit is 'hidden' among other notes, but it has nowhere else to go.
Worked example
5
3
7
6
1
9
4
8
2
1
How to apply it
Check each empty cell in that unit: could the digit legally go there?
If exactly one cell in the unit can hold it, place it there, regardless of that cell's other candidates.
Sweep every digit through every unit; hidden singles are easy to miss without pencil marks.
When to use it
Hidden singles unlock progress when naked singles dry up. They are the workhorse of medium and hard puzzles, and pencil marks make them far easier to spot.
Frequently asked questions
How is a hidden single different from a naked single?
A naked single is a cell with one candidate. A hidden single is a digit with one possible cell in a unit, even if that cell shows several candidates.