Naked Single
A Naked Single means one cell has exactly one remaining candidate. It is the most important beginner technique and should be checked continuously throughout a solve.
This page uses D7-style coordinates (row letter + column number). Where useful, we also
mention legacy notation such as R4C7 for clarity.
When to use it
- At every stage of the puzzle, from opening to endgame.
- Immediately after any elimination from another technique.
- As your first rescan step when progress stalls.
Steps
- Check digits already used in the row, column, and 3x3 box.
- Remove impossible candidates from the target cell.
- If one candidate remains, place it immediately.
Practical scan order
- Start with rows that already have many filled cells.
- Then scan dense columns from left to right.
- After each placement, rescan only connected row/column/box first.
This order reduces full-grid rescans and makes Singles easier to chain.
Concrete example (valid)
Focus on D7 (R4C7) with candidates {2,6,9}. If row and column constraints remove
2 and 9, then only 6 remains and the cell is forced.
Blue marks the final candidate. Red cells provide elimination reasons for 2 and 9.
Invalid case (do not place)
If a cell still has {2,6}, it is not a Naked Single. Do not place until one more elimination is logically justified.
Two candidates remain, so Naked Single cannot be applied yet.
Common mistakes
- Checking row and column but skipping the 3x3 box.
- Forgetting to update neighboring candidates after placement.
- Placing without confirming why the candidate set became size one.