Hidden Single
A Sudoku solving technique · level 1 · first needed around SE grade 1
By The Hardest Sudoku Team · Last updated
TL;DR
A Hidden Single is a digit that can legally go in only one cell within a row, column or box, even if that cell has other candidates.
How does the Hidden Single work?
You look at a single unit (a row, column or box) and a single digit. If that digit can be placed in just one cell of the unit, it must go there, regardless of what other candidates that cell holds.
When does it apply?
When scanning a unit for a digit reveals exactly one viable home for it.
A worked example
In a box, if the digit 7 is blocked from eight of the nine cells by 7s elsewhere in the relevant rows and columns, the ninth cell is the only place 7 can live — a Hidden Single.
See where this fits on the difficulty scale on the SE grade page, or browse every technique in the glossary.