Naked Single
A Naked Single is the simplest deduction in sudoku: a cell where every digit but one has been ruled out. If a cell's row, column and box between them already contain eight of the nine digits, only one candidate remains — and the cell must be that digit. There is nothing hidden about it; the single candidate sits alone in the cell for anyone keeping pencil marks.
Spotting one is a matter of counting. Take an empty cell and run through the digits its peers already hold. Suppose r5c5 is empty, row 5 contains 1, 3 and 8, column 5 contains 2, 4 and 6, and the central box contains 5 and 9. That accounts for eight digits, leaving only 7 — so r5c5 is 7. No further reasoning is needed.
With full pencil marks the technique is even easier: any cell showing exactly one small number is a Naked Single. This is why disciplined candidate-keeping pays off — each placement you make trims the marks of up to twenty peers, and those trims routinely create the next single.
Reach for Naked Singles constantly. They are the bread and butter of every solve, and after any harder deduction the first thing to check is whether it has reduced some cell to a single candidate.
- Scan cells that sit at busy crossings — a full row meeting a full column often leaves just one gap in the count.
- After every placement, re-check the placed cell's row, column and box for new singles.
- If you keep pencil marks, a lone mark in a cell is always safe to place immediately.