Naked Single
A Sudoku solving technique · level 1 · first needed around SE grade 1
By The Hardest Sudoku Team · Last updated
TL;DR
A Naked Single is a cell that has only one remaining candidate, so that value must go there.
How does the Naked Single work?
After eliminating every digit already present in the cell's row, column and 3x3 box, exactly one candidate survives. With no alternative possible, the survivor is placed.
When does it apply?
Whenever a cell's candidate list collapses to a single value — the most fundamental forced placement.
A worked example
If a cell's row already contains 1,2,3,4,5, its column adds 6,7,8 and its box adds 9, then no digit is free except — wait, all nine are taken means a contradiction; but if exactly eight are blocked, the one unblocked digit is the Naked Single.
See where this fits on the difficulty scale on the SE grade page, or browse every technique in the glossary.