Technique
When You Need an Hidden Single
Hidden singles are what separate easy puzzles from medium ones. They are invisible if you only scan cell by cell, because the cell in question still looks ambiguous — you have to scan digit by digit instead.
At a glance
Hidden Single at a Glance
- Technique
- Hidden Single
- Difficulty level
- Beginner
- Also known as
- unique candidate
- First needed on
- Medium puzzles
How to spot it
How to Spot an Hidden Single
Take one digit and one unit, say the digit 7 and a single 3×3 box. Cross off every cell in that box where 7 is blocked by an existing 7 in the same row or column. If exactly one cell survives, that is a hidden single.
Common mistake
The Mistake Solvers Make
Scanning cells rather than digits. Hidden singles are found by asking 'where can this digit go?' not 'what can go in this cell?'. Solvers who only ask the second question stall on medium puzzles.
Practice
Practise This Technique
Medium puzzles are where the hidden single first becomes necessary. Generate a free printable medium Sudoku PDF and work through a few.
More techniques
Other Sudoku Techniques
- Naked Single — beginner
- Naked Pair — intermediate
- Pointing Pair — intermediate
- X-Wing — advanced
- Swordfish — advanced
FAQ
Hidden Single — Frequently Asked Questions
What is an hidden single in Sudoku?
A hidden single is a digit that can legally go in only one cell within a row, column, or 3×3 box — even though that cell may have several other candidates of its own. The digit has nowhere else to go, so it must go there.
How do you find an hidden single?
Take one digit and one unit, say the digit 7 and a single 3×3 box. Cross off every cell in that box where 7 is blocked by an existing 7 in the same row or column. If exactly one cell survives, that is a hidden single.
Is the hidden single a beginner or advanced technique?
It is a beginner technique. Hidden singles are what separate easy puzzles from medium ones. They are invisible if you only scan cell by cell, because the cell in question still looks ambiguous — you have to scan digit by digit instead.