namlabsWhat is a naked single?
Every empty cell has 20 peers: the other cells in its row, its column and its box. A digit placed in any peer cannot go in the cell. When the peers between them already use eight different digits, only one digit is left. That cell is a naked single: the answer is in plain sight.
Example
Look at R5C5, the centre cell. Its row already has 1, 2 and 3. Its column has 4, 5 and 6. Its box has 7 and 8. That is eight different digits, so the only candidate left is 9.
How to find naked singles
- Pick an empty cell in a crowded area: a row, column or box that is nearly full.
- Go through 1 to 9 and check whether each digit already appears in the cell's row, column or box.
- If exactly one digit is missing from all three, write it in.
With pencil marks, naked singles are even easier: any cell with a single candidate left is one. That is why it pays to erase candidates every time you place a digit.
Where it fits
Naked singles and hidden singles solve most easy puzzles on their own. When neither turns up, it is time for pencil marks and naked pairs.
Practise on your phone
Our free Sudoku app has hints that point out the technique you can use next, with the cells highlighted. It plays offline.
