Naked single in Sudoku

How it works

Every empty cell sees twenty other cells: eight in its row, eight in its column, and four more in its box. If those twenty cells contain eight different digits between them, the ninth is the only one left for the cell. No single unit has to be nearly full. The three units do the work together.

1. Pick an empty cell that sits in a crowded row, column and box.
2. Go through 1 to 9. Tick off each digit you can find in the row, the column or the box.
3. If exactly one digit is left unticked, place it.

A real example

The tinted cells are the row, column and box of the green cell. Between them they hold eight different digits, so the green cell has a single candidate left.

Why naked singles feel harder

Nothing about a naked single is complicated, but it is slow to find by eye. You have to check three units for one cell, and there is no pattern on the board that points at it. That is why Sudoku Clan rates a puzzle that needs naked singles as Hard, even though the logic is simple.

Pencil marks change that completely. If you write the possible digits in each cell, a naked single is simply a cell with one note in it. In the game, turn on notes and the single candidate stands out at once.

Where to look

• Cells where a crowded row crosses a crowded column.
• Cells in a box that is already mostly full.
• Any cell whose row, column and box you just added a digit to.