Beginner

Naked Single

A cell with only one possible candidate left — the most fundamental solving move in Sudoku.

A naked single is a cell where eight of the nine digits are already ruled out by its row, column, and box, leaving exactly one candidate. It is the simplest deduction in Sudoku and the move every solve eventually reduces to.

Spotting naked singles quickly is the foundation of fast solving: every more advanced technique exists only to create naked (and hidden) singles elsewhere on the board.

How to spot it

Pick an empty cell and scan its three companions — the row it sits in, the column it sits in, and the 3×3 box it belongs to. Cross off every digit that already appears in any of them. If a single digit survives, that digit must go in the cell.

  • Scan row, column, and box for the empty cell.
  • Eliminate every digit that already appears in those units.
  • If exactly one digit remains, place it.
A naked single up close: every digit the cell can see is crossed off, until one survives.

Why it always works

Each unit (row, column, box) must contain the digits 1–9 exactly once. If eight of those digits are already visible from a cell, the ninth is forced — there is nowhere else in that intersection of constraints for it to go.

Worked example

  1. Choose an empty cell in the top-left box.
  2. Its row already contains 1, 2, 4 and 7.
  3. Its column adds 3, 5 and 8.
  4. Its box adds 9.
  5. Only 6 is left unaccounted for, so the cell is a naked single: place 6.

Try it yourself

7
3
4
5
2
9
6
3
2
1
5
8
7
1
6
4
7
1
4
3
3
4
3
8
4
1
3
1
9
1
7
5
2
6
5
2
3
8

Tap a cell, then a number, to practise.

Frequently asked questions

What is the difference between a naked single and a hidden single?
A naked single is a cell with only one candidate. A hidden single is a digit that can go in only one cell of a unit, even though that cell still appears to have several candidates.
Do I need pencil marks to find naked singles?
No. Naked singles can be found by direct scanning, but full candidate notes make them obvious — a cell showing a single pencil mark is a naked single.

Related techniques

Practice: Naked Single

Put the Naked Single to work on a live board — free puzzles with notes, hints and four difficulty levels.

Try it on a live board

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