Naked single
Only one digit will fit in the cell.
A naked single is a cell whose row, column, and box collectively rule out eight of the nine digits. The ninth has nowhere else to go — write it in.
It's the most basic placement in sudoku, and it's where every solver starts. Easy puzzles can usually be finished with naked singles alone; tougher puzzles depend on them once the elimination work is done.
Practical tip: every time you place a digit, look again at the cells that share its row, column, and box. A cell that had two candidates a moment ago often becomes a naked single after a single placement nearby.
When the move applies
Watch the densest neighborhoods. Rows, columns, and boxes that are nearly filled in are the most fertile ground — every additional placement there has a good chance of forcing a cell down to a single candidate.
The procedure
- Pick any empty cell.
- Cross off every digit that already appears in the cell's row, column, or box.
- If exactly one digit survives, place it.