Naked single

A naked single is a cell that has only one possible candidate. Every other digit has already appeared somewhere in the cell's row, column, or box — so the remaining digit must go here.

It's the most fundamental move in sudoku. When you scan an empty cell and find that eight of the nine digits are already accounted for in its peers, the ninth digit is forced.

Most easy and medium puzzles are solvable using nothing but naked singles and hidden singles. Spotting them quickly is the foundation of fluent solving.

When to look for it

Look for naked singles in densely-populated regions: nearly-complete rows, columns, and boxes. After you place any digit, scan that row, column, and box again — a cell that was a two-candidate cell a moment ago may have just become a naked single.

How to apply it

1. Pick an empty cell.
2. List its candidates: the digits 1-9 that don't yet appear in its row, column, or box.
3. If only one digit remains, place it.

Example