Hidden Single
Beginner Places a digit Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
Instead of looking at one cell, look at one digit across a whole row, column, or box. If every empty cell but one is blocked from holding that digit, it is forced into the cell that remains — even though that cell still shows several pencil marks. The digit is “hidden” among other candidates until you scan the unit.
How to spot it
Pick a digit and a unit (row, column, or box); check whether every empty cell but one is blocked from taking it.
What you can conclude
Place the digit in the single cell that can still hold it.
Worked example
Across this row, every empty cell but one is already blocked from holding 4, so the lone remaining cell must take it — even though it still shows other candidates.