Hidden Single
A digit has only one possible cell left in a row, column, or box — even if that cell has other candidates.
How it works
- 1Pick a digit and a house, and look at every cell in that house that could still take it.
- 2If there is exactly one, that cell must hold the digit — whatever else its own pencil marks still say.
- 3This fires constantly on a Killer board because cage arithmetic prunes candidates unevenly, often leaving a digit just one home.