Difficulty
Beginner
Sudoku technique guide
Naked Single
Most Sudoku puzzles begin to open up once you know what to notice. Naked 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, and throughout a puzzle
Frequency
Very common
Prerequisites
The basic row, column and box rule
Why this technique matters
Naked Singles are the first dependable way to turn scanning into a solved square. Once you can spot them, every partly filled grid becomes easier to read.
The idea in plain English
A Naked Single is an empty cell with only one possible number left. No other number can fit there, so that candidate must be the answer.
How to recognise it
Look for cells boxed in by many given numbers. If you use pencil marks, a cell showing one candidate is a Naked Single.
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. Choose an empty cell and check which numbers are already used in its row.
Then. Cross off numbers used in its column.
Next. Cross off numbers used in its 3x3 box.
Finally. If one number remains, write it in. Then rescan the row, column and box it affects.
When it usually appears
Use it constantly while solving. A new placement often creates another single nearby, so rescan after every answer.
Common traps
These are easy places to get turned around. They are useful checks, not signs that you are doing badly.
- Forgetting the box. A number may be absent from the row and column but already present in the 3x3 box.
- Stopping after one scan. The first single you place can immediately create another one.
Frequently asked questions
Is a Naked Single a guess?
No. Every other number has been ruled out by the Sudoku rules.
Do I need pencil marks?
No, but they make singles much quicker to spot in a harder grid.
Quick takeaway
- A Naked 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, and throughout a puzzle.
- Once you make the deduction, return to the simpler scans.