Sudoku technique · First needed at: Fast ●○○○○○○○

Naked single

Only one number still fits in this cell.

An example from a real puzzle

How to spot it

1. Pick an empty cell and look at the numbers already in its row, its column and its box.
2. If eight different numbers are there, the ninth is the only one the cell can hold.

Why it works

Every row, column and box needs each number exactly once, so a number already in any of the three can never go in this cell.