Naked single

Medium

A naked single is a cell where only one digit is left: its row, column and box already hold the other eight. Medium puzzles need it, because some of their cells can’t be found as hidden singles.

How it works

Pick an empty cell and collect the digits its row, column and box already hold between them. If eight different digits turn up, the ninth is the only one left, and it goes in the cell.

Notes make them easy to see

From Medium puzzles on, it pays to write notes: the digits each empty cell can still take. A naked single is then simply a cell with one note left. In Sudoku Knockout, switch on Notes and pencil them in, and turn on Auto-remove notes in the settings so that every digit you place clears itself from the notes around it.

Notes are also what every harder technique works on: from here on, they’re all about ruling notes out.

How to spot them

• Look where a busy row crosses a busy column: a cell that sees many placed digits has few left.
• After placing a digit, check the cells around it that had two notes: one of them may be down to one.
• When you can’t find a hidden single anywhere, a naked single is usually the next step.