Naked Single
A cell with one and only one possible digit. The easiest move in sudoku and the most common.
Beginner sudoku technique
What it is
A naked single is a cell whose row, column, and 3×3 box together have already used eight of the nine digits — leaving exactly one digit that can legally go there. Place it. Naked singles drive almost every easy-tier puzzle and continue to appear throughout medium and hard puzzles as the board fills in. Spotting them is the single most important skill to develop; every other technique exists to create more naked singles down the line.
When to use it
Scan first, always. Before reaching for any advanced technique, sweep each row, column, and box for cells with a single remaining candidate. A naked single might appear at any moment in the solve as new digits get placed.
Worked example
Suppose row 4 already has 1, 2, 3, 4, 5, 6, 7, 8 placed. The one empty cell in row 4 must be 9 — no other digit is legal. That's a naked single in its simplest form. A subtler example: cell R3C5 sits in a row containing {2, 4, 7}, a column containing {1, 5, 6}, and a box containing {3, 8}. Together those rule out every digit except 9. R3C5 must be 9.
Try it
The Sudoku247Online solver walks you through every move of any puzzle one logical step at a time, naming the technique that justifies each placement. Paste a puzzle to see this technique applied in real time.