Tier 1 · first asked for by Gentle
Naked Single
A cell with only one candidate left must hold that digit.
Also called Sole Candidate, Forced Digit.
The simplest deduction there is. Look at an empty cell and cross off every digit already placed in its row, its column and its box. If exactly one digit survives, that cell must hold it — there is nowhere else for it to go and nothing else it can be.
Most of any puzzle is made of these. They are also what a placement cascades into: filling one cell removes a candidate from twenty others, which often leaves one of those with a single option in turn.
A worked example
A real position, with the pencil marks a solver would have at that point.
the pattern · what it places
What it proves. R9C7 has only one candidate left, so it must be 9.