Naked Single
The simplest move. A cell where every digit except one has already been claimed by its row, its column, or its box.
3
6
7
1
2
8
9
4
Walk it through
Look at the highlighted cell in the centre. Eight of the nine digits 1–9 are already ruled out:
- Row 5: already has 1 and 2.
- Column 5: already has 3 and 4.
- Centre box: already has 6, 7, 8, 9.
That accounts for 1, 2, 3, 4, 6, 7, 8, 9 — eight digits gone. Only 5 remains. The cell can only be 5.
Spotting them
Naked Singles often appear after you've filled a few digits. Scan cells with the most constraints — one row near complete, or a tightly populated box — and check whether only one digit fits.