What it is
Take an empty cell and cross off every digit that already appears in its row, its column or its box. If exactly one digit survives, that digit belongs in that cell. There is nothing more to it — no pattern to recognise and no other cell to consult.
The highlighted cell has only one candidate left: every other digit already appears in its row, its column or its box.
Finding them without checking all 81 cells
Naked singles cluster where the board is already crowded. Rather than sweeping the grid, look at:
- Boxes with seven or eight digits already placed.
- Rows and columns with one or two empty cells left.
- The cells around a digit you have just placed — a placement often creates a naked single next to it.
Naked single or hidden single?
They answer different questions. A naked single asks *which digits can go in this cell* and finds one answer. A hidden single asks *where can this digit go in this unit* and finds one cell. Both place a digit; each finds moves the other misses, which is why good solvers alternate between them.
The trap
Naked singles are the technique most damaged by careless pencil marks. If you forget to erase a candidate after placing a digit, a cell can look like it has two candidates when it has one — or worse, look solved when it is not. Clear candidates from every affected cell the moment you place a digit; on this site the notes tool does that for you.
Practise on an easy puzzle, where singles alone are enough to finish.
Common questions
What is a naked single in Sudoku?
An empty cell where only one digit remains possible after eliminating everything that appears in its row, column and box. That digit can be placed immediately.
Is a naked single the same as a sole candidate?
Yes — sole candidate, forced digit and singleton are all names for the same move.
Do I need pencil marks to find naked singles?
Not on easy puzzles, where they are usually visible by eye. On harder grids candidate marks make them much quicker to see.