Solving techniques
Naked single
The naked single is the simplest idea in Sudoku: look at one cell, rule out the digits its row, column and box already use, and see if one digit remains.
A naked single is an empty cell where eight of the nine digits already appear in its row, column or box, so only one candidate is left. That digit must go in the cell. It is the first technique every solver learns.
How it works
Every empty cell sees 20 other cells: the rest of its row, its column and its box. Any digit placed in those cells cannot go in this one. If the cells it sees already contain eight different digits, only the ninth can fit.
How to find naked singles
- Pick a cell in a crowded row, column or box.
- List the digits the cell can still hold: cross out every digit in its row, column and box.
- If one digit is left, place it.
- With notes written, simply look for a cell whose note has exactly one digit.
Naked single or hidden single?
They sound alike but look from different sides. A naked single is about a cell that has only one digit left. A hidden single is about a digit that has only one cell left in a unit. A good solver looks for both. Read the full list of techniques to see where they fit.
Common mistakes
- Forgetting a digit that is in the same box but a different row and column.
- Not updating notes after placing a digit, so a single stays hidden in the clutter.
- Placing the digit without checking the cell is truly empty of other options.
Practice
Naked singles are all you need for most Easy puzzles. Try one in the online game and use Hint when you are stuck.
Frequently asked questions
What is a naked single in Sudoku?
An empty cell with only one candidate digit left, so that digit must go there.
Why is it called naked?
Because the single candidate is plainly visible in the cell. In a hidden single, the digit is hidden among other candidates.
Do I need notes to find naked singles?
Not on Easy puzzles, but notes make them much faster to see.