Naked Single Walkthrough
A Naked Single is a cell left with only one possible candidate once the digits already in its row, column, and 3x3 box are eliminated. That digit must go there.
There is a Naked Single in Row 2, Column 1. Applying the Naked Single eliminates 291 candidates and solves 5 cells (struck out in red below; any solved cells shown in green).
Reading this board
The pattern on this puzzle involves row 2. Work from the before-image above: find the naked single for yourself before scrolling to the after-image, since the point of a worked example is the search, not the answer. The eliminations here immediately resolve a cell, so this move both cleans the grid and produces a placement you can build on.
Why the deduction is valid
A Naked Single is the point where elimination has already done all the work. Every digit but one has been ruled out of the cell by something visible in its row, its column, or its box, and Sudoku guarantees each cell holds exactly one digit. So the survivor is not a good guess — it is the only legal occupant, and writing it in is a certainty rather than a commitment.
How to spot a Naked Single on your own board
- Work the cells with the fewest pencil marks first. If you keep candidate notation at all, a Naked Single is literally a cell displaying one mark, so it is found by looking rather than reasoning.
- After every digit you place, re-check the three units that digit sits in. Placing a digit removes a candidate from up to twenty other cells, and the cell that was down to two marks is now down to one.
- Without pencil marks, scan the fullest rows, columns and boxes. A unit with seven digits already placed leaves each empty cell in it with very few survivors.
Common mistakes
- Hunting for exotic patterns while Naked Singles sit unclaimed. Advanced techniques exist to create singles; if singles are already available, the advanced move is wasted effort.
- Forgetting that a placement cascades. Solvers often place one digit, then resume scanning from the top of the grid instead of from the cell they just filled, which is where the next single almost always is.
What to do next
Placing a Naked Single usually creates more of them. Follow the chain until it dries up completely before you go looking for anything harder — most easy and medium puzzles are nothing but one long cascade of singles, and even the hardest puzzles collapse into a cascade once their one difficult step is found.