Sometimes you stare at one empty cell and ask: “What could this cell still be?”
Its row already forbids some digits. Its column forbids more. Its box forbids even more. Shrink those possibilities further… and further… until only one digit survives. That cell is a Naked Single.
Why it works
A cell must become exactly one of 1–9. If eight digits are impossible (because they already appear in the same row, column, or box), then the ninth digit is the only survivor.
“Naked” means you can see it by looking at that cell’s candidates — opposite of Hidden Single, where you ask *where can this digit go?*
How to spot it
Scan cells that already see many big digits — their candidate lists shrink fastest.
Demonstration
Focus on one empty cell. It must be some digit from 1–9. Start with every digit still possible.
Anything already in the same row is banned. Cross those off. The list shrinks further…
…and further when you check the column and the box. Peers keep deleting options.
When the pencil marks shrink until one number remains, you have a Naked Single. The cell has nowhere else to hide.
Place 8 here. You did not guess — every other digit was already ruled out by the three houses that touch this cell.
Example 1
Scan empty cells for one whose candidates have almost vanished.
This cell has only one candidate left — a Naked Single.
Place 4. It is the only digit that still fits after peers banned the rest.
Example 2
Scan empty cells for one whose candidates have almost vanished.
This cell has only one candidate left — a Naked Single.
Place 4. It is the only digit that still fits after peers banned the rest.