Sudoku techniques · Beginner level

Naked singles: the cell with one digit left

A naked single is an empty cell that can only hold one digit, because every other digit already appears in its row, its column or its box. That digit goes in the cell.

How it works

Every empty cell sees twenty others: the rest of its row, its column and its box. Collect the digits in those cells. If they cover eight of the nine digits, the ninth is the only one left for your cell.

It is called naked because nothing hides it: the answer sits in plain view once you count. It is the simplest move in Sudoku, and the one you will use most. Ninebloom's Beginner puzzles need nothing else.

The row, column or box the move uses
The cells that make the pattern
Candidates the move removes

Naked single example on a Sudoku board: R5C8 already sees the other eight digits through its row, column and box. Only 7 is left, so it goes here. — Example 1 · from Beginner puzzle #1

Cross off what R5C8 can see: row 5 has 1, 2, 3, 4, 5, 6, 8 and 9. That is every digit except 7, so **R5C8 is 7**.

Play this position in Ninebloom: **Home › Your own puzzle**, then paste the code. `NG-1RRYYP-ITGDQ5-09TMZQ-M2SCNI-DSKC3E-9IHLLE-0RLV5G-97ESD4-YN7K7`

Cells are named by row and column: R4C3 is row 4, column 3. Boxes are numbered 1 to 9, left to right and top to bottom.

How to spot it

Start with the busiest row, column or box, where few cells are empty.
For one empty cell, list the digits in its row, then add the ones in its column, then its box.
If only one digit from 1 to 9 is missing from your list, write it in.
Each digit you place fills a gap in three units, so look again at the cells around it.

More examples

Naked single example on a Sudoku board: R9C3 already sees the other eight digits through its row, column and box. Only 5 is left, so it goes here. — Example 2 · from Beginner puzzle #2

Cross off what R9C3 can see: row 9 has 1, 2, 3, 6 and 9; column 3 adds 4, 7 and 8. That is every digit except 5, so **R9C3 is 5**.

Play this position in Ninebloom: **Home › Your own puzzle**, then paste the code. `NG-16TIB2-ZAE83G-Q170HI-2SUJRF-Q2LJO5-83N6X8-CYTOLV-0VWIMP-GEOHY`

Common mistakes

Checking only the row and column. The box counts too, and it is the one most often forgotten.
Stopping at seven. You need eight different digits seen before the cell is settled.
Mixing it up with a hidden single, which starts from a digit and asks where it can go.

Questions

What is the difference between a naked single and a hidden single?

A naked single is a cell that can only hold one digit. A hidden single is a digit that can only go in one cell of a row, column or box. The first looks at a cell, the second looks at a digit.

Do I need pencil marks to find naked singles?

No. Counting the digits a cell can see is enough. Pencil marks help on harder puzzles, where a single only appears after other moves have removed candidates.

Which Ninebloom level uses naked singles?

Beginner. A Beginner puzzle can be solved with naked singles alone, and every level above uses them too.