Intermediate
Obvious Singles
A core Sudoku solving technique for intermediate players
What are Obvious Singles?
An Obvious Single (also called a Naked Single) is a cell that has only one possible candidate remaining. After you write out all pencil marks for a cell, if only one digit is left - That digit is the answer.
The "obvious" label comes from the fact that the answer is directly visible in the candidate list with no further logic needed. Compare this to Hidden Singles, where the cell appears to have multiple candidates but only one of those candidates appears anywhere in the unit.
Obvious Singles vs Last Free Cell
| Last Free Cell | Obvious Singles | |
|---|---|---|
| Trigger | Unit has 1 empty cell | Cell has 1 candidate |
| Notes needed | No | Yes (or careful counting) |
| Typical level | Easy | Medium+ |
| Reason it works | 8 cells are filled, one left | 8 digits are eliminated, one left |
Step-by-step guide
- Fill in candidate notes for every empty cell.
- Scan cells for any whose note set contains exactly one digit.
- Place that digit and clear the cell's notes.
- Update notes in all peers (same row, column, box) - Remove the placed digit from their candidates.
- Check if the update created new Obvious Singles and repeat.
Frequency by difficulty
Approximate % of cells solvable by Obvious Singles alone.
Pro tip: After placing an Obvious Single, immediately re-scan its peers. A single placement often creates a cascade of new Obvious Singles, especially on Easy and Medium puzzles.