Forgetting to check one of the three constraints: row, column, or box.
Sudoku technique · Foundation
Naked Single
A Naked Single is an empty cell with exactly one legal candidate after its row, column, and box are considered.
Learning order
Topic relationships
These links express the same learning and proof relationships used throughout the library.
- Teaching aliasLast Remaining Cell
- Uses candidate language fromCandidate Notes
Verified example
Work through the example
R1C1: given 11 | R1C2: given 22 | R1C3: given 33 | R1C4: given 44 | R1C5: given 55 | R1C6: given 66 | R1C7: given 77 | R1C8: given 88 | R1C9: given 99 |
R2C1: given 44 | R2C2: given 55 | R2C3: given 66 | R2C4: given 77 | R2C5: given 88 | R2C6: given 99 | R2C7: given 11 | R2C8: given 22 | R2C9: given 33 |
R3C1: given 77 | R3C2: given 88 | R3C3: given 99 | R3C4: given 11 | R3C5: given 22 | R3C6: given 33 | R3C7: given 44 | R3C8: given 55 | R3C9: given 66 |
R4C1: given 22 | R4C2: given 33 | R4C3: given 44 | R4C4: given 55 | R4C5: given 66 | R4C6: given 77 | R4C7: given 88 | R4C8: given 99 | R4C9: given 11 |
R5C1: given 55 | R5C2: given 66 | R5C3: given 77 | R5C4: given 88 | R5C5: target candidates; Before 9 | R5C6: given 11 | R5C7: given 22 | R5C8: given 33 | R5C9: given 44 |
R6C1: given 88 | R6C2: given 99 | R6C3: given 11 | R6C4: given 22 | R6C5: given 33 | R6C6: given 44 | R6C7: given 55 | R6C8: given 66 | R6C9: given 77 |
R7C1: given 33 | R7C2: given 44 | R7C3: given 55 | R7C4: given 66 | R7C5: given 77 | R7C6: given 88 | R7C7: given 99 | R7C8: given 11 | R7C9: given 22 |
R8C1: given 66 | R8C2: given 77 | R8C3: given 88 | R8C4: given 99 | R8C5: given 11 | R8C6: given 22 | R8C7: given 33 | R8C8: given 44 | R8C9: given 55 |
R9C1: given 99 | R9C2: given 11 | R9C3: given 22 | R9C4: given 33 | R9C5: given 44 | R9C6: given 55 | R9C7: given 66 | R9C8: given 77 | R9C9: given 88 |
R5C5 has derived candidates 9. Place 9.
- Target cell
- R5C5
- Derived candidates
- 9
- Verified result
- Place 9
- Reviewed unit
- —
- Start with the digits 1 through 9 for the target cell.
- Remove every digit already present in the same row.
- Remove every digit already present in the same column and 3×3 box.
- If one candidate remains, place it and update the related notes.
Fixed practice
Ignore existing pencil marks and recompute the target cell from 1 through 9 using its row, column, and box.
Fixed practice · R1C1
R1C1: target candidates; Before 1 | R1C2: given 22 | R1C3: given 33 | R1C4: given 44 | R1C5: given 55 | R1C6: given 66 | R1C7: given 77 | R1C8: given 88 | R1C9: given 99 |
R2C1: given 44 | R2C2: given 55 | R2C3: given 66 | R2C4: given 77 | R2C5: given 88 | R2C6: given 99 | R2C7: given 11 | R2C8: given 22 | R2C9: given 33 |
R3C1: given 77 | R3C2: given 88 | R3C3: given 99 | R3C4: given 11 | R3C5: given 22 | R3C6: given 33 | R3C7: given 44 | R3C8: given 55 | R3C9: given 66 |
R4C1: given 22 | R4C2: given 33 | R4C3: given 44 | R4C4: given 55 | R4C5: given 66 | R4C6: given 77 | R4C7: given 88 | R4C8: given 99 | R4C9: given 11 |
R5C1: given 55 | R5C2: given 66 | R5C3: given 77 | R5C4: given 88 | R5C5: given 99 | R5C6: given 11 | R5C7: given 22 | R5C8: given 33 | R5C9: given 44 |
R6C1: given 88 | R6C2: given 99 | R6C3: given 11 | R6C4: given 22 | R6C5: given 33 | R6C6: given 44 | R6C7: given 55 | R6C8: given 66 | R6C9: given 77 |
R7C1: given 33 | R7C2: given 44 | R7C3: given 55 | R7C4: given 66 | R7C5: given 77 | R7C6: given 88 | R7C7: given 99 | R7C8: given 11 | R7C9: given 22 |
R8C1: given 66 | R8C2: given 77 | R8C3: given 88 | R8C4: given 99 | R8C5: given 11 | R8C6: given 22 | R8C7: given 33 | R8C8: given 44 | R8C9: given 55 |
R9C1: given 99 | R9C2: given 11 | R9C3: given 22 | R9C4: given 33 | R9C5: given 44 | R9C6: given 55 | R9C7: given 66 | R9C8: given 77 | R9C9: given 88 |
R1C1 has derived candidates 1. Place 1.
- Target cell
- R1C1
- Derived candidates
- 1
- Verified result
- Place 1
- Reviewed unit
- —
Common mistakes
Common mistakes
Trusting a single written note without confirming that the notes are complete and current.
Review information
What is verified here
The verifier recomputes the target cell’s complete legal candidate set and accepts the move only when that set contains exactly one digit.
- Fixture ID
- p2-naked-single-02 / p2-naked-single-01
- Engine version
- legen-sudoku-engine/1.3.0
- Verification status
- Independent placement verifier passed
- Facts reviewed
- 2026-07-18
Technique terminology and examples are tied to the published engine version. See the review information below for scope and dates.