Naked single
A naked single is an empty cell that has exactly one candidate: the other eight digits already appear in its row, column or box. The remaining digit must go there. It is the simplest Sudoku technique, the one you use most, and the one the Bare Sudoku hint reports as "Only one candidate here".
When to use it
Always. Naked singles appear from the first move to the last, at every level. In Easy and Medium puzzles they are, together with hidden singles, the only technique you need. In harder puzzles every elimination made by an advanced technique usually leaves a naked single behind, so check for them after every step.
How to spot it
Look at cells with many placed peers: a cell in a row that already has seven digits, or a cell whose row, column and box together cover eight different digits. Without notes, count the digits you can see from the cell. With notes, a naked single is simply a cell showing one candidate, which is why Fill notes makes them easy to find.
Example
| c1 | c2 | c3 | c4 | c5 | c6 | c7 | c8 | c9 | |
|---|---|---|---|---|---|---|---|---|---|
| r1 | 145 | 15 | 2 | 145 | 6 | 148 | 3 | 59 | 7 |
| r2 | 7 | 135 | 1 | 135 | 2 | 9 | 4 | 6 | 8 |
| r3 | 456 | 9 | 8 | 345 | 345 | 47 | 1 | 5 | 2 |
| r4 | 9 | 68 | 67 | 46 | 48 | 5 | 2 | 1 | 34 |
| r5 | 156 | 12568 | 16 | 7 | 489 | 3 | 58 | 4589 | 49 |
| r6 | 15 | 4 | 3 | 2 | 89 | 18 | 58 | 5789 | 6 |
| r7 | 8 | 16 | 4 | 569 | 59 | 26 | 7 | 3 | 1 |
| r8 | 3 | 7 | 9 | 8 | 1 | 246 | 6 | 24 | 5 |
| r9 | 2 | 16 | 5 | 346 | 7 | 46 | 9 | 48 | 14 |
Bold digits are given, blue digits were placed while solving, small digits are candidates. Highlighted cells form the pattern, crossed-out candidates are removed, and a circled candidate is the digit to place.
- Look at r2c3. It is empty, so list every digit that is already placed in its row, its column and its box.
- Row 2 contains 2, 4, 6, 7, 8, 9. Column 3 contains 2, 3, 4, 5, 8, 9. Box 1 contains 2, 7, 8, 9.
- Together they rule out every digit except 1, so 1 is the only candidate left in r2c3.
- Write 1 in r2c3. Then check whether the new digit creates another single in the same row, column or box.
Common mistakes
- Counting only the row or only the box. The eight excluded digits usually come from all three units together.
- Forgetting that a digit you placed a moment ago also excludes candidates from its 20 peers; new naked singles appear right after each placement.
- Confusing a naked single with a hidden single: a naked single has one candidate left in the cell, a hidden single has one cell left for the digit in a unit.
In Bare Sudoku
Needed from level: Easy
In the game, the hint says: Only one candidate here: 1
Frequently asked questions
What is a naked single in Sudoku?
An empty cell with exactly one candidate. Eight different digits are already placed in its row, column and box, so the ninth digit is the only one that fits.
Why is it called naked?
Because the single candidate is visible directly in the cell once the notes are written; nothing hides it. A hidden single, by contrast, sits among other candidates and is found by looking at the digit rather than the cell.
Can a puzzle be solved with naked singles only?
Many Easy puzzles can, but most also need hidden singles. Bare Sudoku rates Easy and Medium puzzles as solvable with singles of both kinds; Hard and Expert puzzles need at least one further technique.
Does the hint show naked singles?
Yes. When the next placement is a naked single, the hint selects the cell and says "Only one candidate here" with the digit. Tap the hint again to place it.