Step 16 · Master
What is BUG+1? How to Solve Sudoku
When every empty cell has exactly two candidates except one cell with three, that cell's answer is fixed — otherwise the puzzle would have two solutions.
What it is
Late in the puzzle, nearly every empty cell has two candidates, and just one cell has three.
If that cell became a two-way choice too, every cell would stay a two-way choice and the puzzle would have two solutions.
So that cell gets the digit that appears three times in its row (column or box).
How to spot it
When almost every cell is down to two candidates, find the one with three and see which of its digits appears three times.
Example
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
Every empty cell has two candidates except R4C5, which has three. 9 is the candidate that appears three times in box 5. If R4C5 weren't 9, every cell would stay a two-way choice and the puzzle would have two solutions. So R4C5 is 9.
→ R4C5 is 9.