Naked Single
A cell has only one candidate left after eliminating peers.
A naked single is the simplest move in naked single sudoku: a cell whose row, column and box between them have already used eight of the nine digits, so exactly one candidate remains. Because you can write the answer in straight away with no chains or extra logic, spotting naked singles is the very first skill a solver learns and the one that finishes off most puzzles once the harder work is done.
Worked example
On this board R1C1 is empty, so check what its own row, column and box have already claimed. The row across R1 already contains 2, 3, 4 and 5, ruling those four out; the top-left box already contains 2, 3, 6, 7, 8 and 9, ruling those out too. Put together, every digit from 2 to 9 is accounted for, which leaves 1 as the only candidate for R1C1 — a naked single — so you place 1 there directly.
When to use it
Reach for naked singles first, before any other technique, and return to them after every single placement — filling one cell often strips a neighbour down to its last candidate. Spot them by picking a cell and mentally striking out every digit that already appears in its row, column or box; the moment a cell drops to one surviving digit, as R1C1 did with only 1 left, you have a naked single to write in. They are easiest to find in rows, columns or boxes that are already nearly full, where seven or eight digits are placed.
How to use it
- Pick an empty cell. Choose any empty cell and look at its row, column, and box.
- Eliminate seen digits. Cross off every digit that already appears in that cell’s row, column, or box.
- One candidate remains. If exactly one digit survives, it is a Naked Single — place it with confidence.
Frequently asked questions
What is a naked single in sudoku?
A naked single is a cell that has just one candidate remaining because the other eight digits already appear somewhere in its row, column or box. Since nothing else can legally go there, you place that digit immediately — on the example board R1C1 is a naked single for 1, because 2 to 9 are all blocked by its row and box.
How do I spot a naked single quickly?
Focus on rows, columns and boxes that are already nearly full, and for each empty cell in them cross off the digits its row, column and box contain. If exactly one digit survives, that cell is a naked single. In the example you only had to combine R1's 2,3,4,5 with the box's 2,3,6,7,8,9 to see that 1 was the sole survivor in R1C1.
What is the difference between a naked single and a hidden single?
A naked single is about one cell: that cell has only one candidate left, like R1C1 which can only be 1. A hidden single is about one unit: a digit has only one cell it can still go in within a row, column or box, even though that cell may still show several candidates. Naked singles are usually the easier of the two to see because you read them straight off a single cell.