Difficulty
Beginner
Sudoku technique guide
Hidden Single
Most Sudoku puzzles begin to open up once you know what to notice. Hidden Single gives you one small, dependable clue you can trust.
By the end, you will know what to notice, how the idea works and when it is worth using.
At a glance
Technique family
Singles
Purpose
Places a number
Typical puzzle stage
Early to middle
Frequency
Very common
Prerequisites
Naked Singles and basic candidate marks
Why this technique matters
Hidden Singles teach you to scan a whole row, column or box instead of staring at one cell. They unlock many grids that look stuck after easy Naked Singles.
The idea in plain English
A Hidden Single occurs when one digit can go in only one cell within a row, column or box. The cell may have several candidates, but the digit has only one possible home.
How to recognise it
Look for a missing digit, not a nearly empty cell. In candidate notes, you are searching for a digit that appears once across a unit.
Pause when you see this shape. You do not have to force it; just check whether the pattern is complete.
How the logic unfolds
Think of this as a short conversation with the grid. Take one check at a time, and let each answer tell you what to inspect next.
Start by. Pick one unit: a row, column or box.
Then. Choose a missing digit and look at every empty cell where it could go.
Next. If that digit appears as a candidate in only one cell, place it there.
Finally. Repeat for the other missing digits before moving on.
When it usually appears
Use this when every empty cell still seems to have two or more options. It is especially useful after a few placements change candidate positions.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Counting candidate cells rather than candidate digits. A Hidden Single is about one digit having one location.
- Checking only rows. A hidden single can be hidden in a column or box too.
Frequently asked questions
Why is it called hidden?
The cell may have more than one candidate, so the answer is hidden until you compare the whole unit.
Can a digit be hidden in more than one unit?
Yes. One placement can be a Hidden Single in its row and its box at the same time.
Quick takeaway
- A Hidden Single belongs to the singles family.
- Its purpose is to place a number.
- Look for a complete pattern, not one that only nearly fits.
- It is most useful early to middle.
- Once you make the deduction, return to the simpler scans.