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

Hidden single

Only one cell in this group can hold this number.

An example from a real puzzle

How to spot it

1. Choose a number and a row, column or box that still needs it.
2. Rule out every cell of that group where the number would clash with its own row, column or box.
3. If only one cell is left, the number goes there — even if that cell still has other candidates.

Why it works

The group has to contain the number somewhere, and every other place in it is ruled out.