Hard Used from hard puzzles up.
BUG+1
One cell away from two solutions
Near the end, every empty cell may hold exactly two candidates but one, which holds three. Count each digit's places in every unit: if each has exactly two, but for one digit of that cell, which has three in its row, its column and its box, the position is a BUG+1 (Bivalue Universal Grave, plus one).
Without that extra candidate, every digit would have two places in each of its units and could take either: the puzzle would have two solutions, or none. A proper Sudoku has one, so the extra candidate is the cell's value: place it.
Like the Unique Rectangle, this relies on the puzzle having a single solution.
What to look for
When every empty cell has two candidates but one with three, place there the digit that has three places in its row.
Example
- The cell it fills, with its digit in green
Practise it
In the app, Learn › BUG+1 sets exercises on real positions, then a full puzzle that needs this technique and nothing harder.