Naked Single
Beginner Places a digit Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
Look at a single empty cell and rule out every digit blocked by the puzzle’s constraints — its row, its column, and any box, cage, or inequality it belongs to. When only one digit survives, that cell is solved — there is literally no other number it can hold. This is the most fundamental move in every grid puzzle.
How to spot it
Scan for a cell whose row, column, and box together already contain eight different digits.
What you can conclude
The ninth digit is the only one left — write it in.
Worked example
If a cell’s row already shows 1, 2 and 3, its column adds 4 and 5, and its box contributes 6, 8 and 9, then 7 is the only digit left — place it.