Sudoku technique · First needed at: Extreme ●●●●●●●●

BUG+1

With every cell down to two candidates a grid would have two answers — so the one cell left with three takes the number that appears three times in a unit.

An example from a real puzzle

How to spot it

1. Every empty cell has two candidates, except one cell with three.
2. In that cell, the number that appears three times in its row, column or box is the answer.

Why it works

With any other value, every cell would be left with a pair and the puzzle would have two solutions — a 'bivalue universal grave', which a proper sudoku never allows.