Hidden Single

A digit fits only one cell within a unit.

Learn the Hidden Single Sudoku technique: a digit that can go in only one cell of a row, column, or box. Free browser Sudoku.

How to use it

1. Pick a digit and a unit. Choose a digit (say 4) and a single row, column, or box that does not yet contain it.
2. Test each empty cell. Mark which empty cells in that unit could legally hold the digit, ruling out cells whose peers already use it.
3. Only one fits. If just one cell can take the digit, place it there — even though the cell has other candidates too.