A Naked Single occurs when a cell has only one possible candidate number. This happens after you have eliminated other candidates based on the numbers already present in the corresponding row, column, and block.
Here's how to identify and solve a Naked Single:
- Find cells with only one possible candidate.
- Place that number in the cell.
- Remove that number as a candidate from cells in the same row, column, and block.
- Repeat the process across the grid.
Row 3, column 3 has only one candidate left: 7 (in blue). The 7s it will clear from its row, column, and block are tinted red.
At row 3, column 3, candidates 3, 1, 5, and 9 are eliminated from the column. In the row, 6, 2, 9, 5, and 8 are already present. The 3x3 block also contains 3, 4, and 6. This leaves 7 as the only possible solution—the Naked Single.
How to spot it
Scan for a cell where eight of the nine digits already appear in its row, column, or box. The one digit left is forced. Pencil marks make it obvious: a cell showing a single candidate is a Naked Single.
When to use it
It is the most basic move and the first thing to look for after every placement, because solving one cell often creates the next Naked Single.
Common mistakes
- Confusing it with a Hidden Single: a Naked Single has only one candidate in the cell; a Hidden Single has other candidates but is the only home for a digit in its unit.
- Forgetting to re-scan the cell's peers after placing it, missing the chain of singles it unlocks.
Frequently asked questions
- What is a Naked Single in Sudoku?
- A Naked Single is a cell that has exactly one candidate left after you remove every digit already present in its row, column, and box. That remaining digit must go in the cell.
- How do you find a Naked Single?
- Look at an empty cell and cross off every digit that appears in its row, column, or 3x3 box. If only one digit survives, you have a Naked Single and can place it immediately.
- What is the difference between a Naked Single and a Hidden Single?
- A Naked Single has only one candidate in the cell itself. A Hidden Single may have several candidates, but one of those digits can go in no other cell of a row, column, or box, so it is forced there.