Naked single

A naked single is a cell on the Sudoku grid that has only one digit left it can legally hold. Place that digit; move on; do it again. It is the most foundational deduction in Sudoku and the first move every player learns. On easy puzzles, naked singles fill in roughly the first 60% of the grid before anything trickier is needed.

How to spot one

Pick an empty cell. Look at every digit already placed in its row, its column, and its 3×3 box — anything in any of those three groups can't go in your chosen cell, because Sudoku doesn't repeat digits within a unit. Mentally cross those digits off the list of 1–9. If exactly one digit survives, that's the naked single, and it belongs in the cell.

The move is "naked" in the sense that the answer is exposed: the cell can only hold one thing, and the thing it can hold is right there to be read. There's no inference about other cells and no pattern that spans multiple units. Just the one cell, three groups, and the digit that hasn't shown up in any of them.

When you'll see it

Naked singles are densest at the start of an easy puzzle, and they cascade — placing one tightens the constraints on its row, column, and box, which often produces a fresh naked single in a cell that was ambiguous a moment earlier. A good easy-puzzle workflow is to scan the grid for naked singles, place every one you find, then scan again. The grid solves itself in waves.

On medium and harder puzzles naked singles still appear, but they run out earlier. The grid stalls, and the player has to switch to a different lens — usually the hidden single, where the question shifts from "what digit goes in this cell" to "where in this row does this digit go."

Why it's the foundation

Almost every Sudoku technique that comes after the naked single is, in some sense, an extension of it. Naked pairs and triples are the same idea applied to two or three cells together. Pointing pairs use locked-candidate logic that builds on counting digits the naked-single way. Even the harder techniques — the X-wing, the swordfish — fall back on the naked-single test once they've made their elimination, because the elimination is only useful when it leaves some cell with one option remaining.

If naked singles feel automatic, the rest of the technique ladder is a series of variations on a move you already make.

A longer walk-through of the naked single alongside its sibling, the hidden single, lives in The two moves that solve most easy puzzles. For where to start scanning on a fresh grid before any singles are visible, see Where to look first on a fresh grid.