Leaving impossible or outdated candidates in the notes.
Sudoku technique · Foundation
Candidate Notes
Candidate Notes are temporary records of the digits currently legal in an empty cell, used to expose forced moves as the board changes.
Learning order
Topic relationships
These links express the same learning and proof relationships used throughout the library.
- Candidate language forNaked Single
- Candidate language forHidden Single
- Candidate language forLocked Candidates
- Candidate language forNaked Pair
- Candidate language forHidden Pair
- Candidate language forNaked Triple
- Candidate language forHidden Triple
- Candidate language forX-Wing
- Candidate language forXY-Wing
- Candidate language forSwordfish
Verified example
Work through the example
R1C1: given 11 | R1C2: given 44 | R1C3: given 55 | R1C4: given 66 | R1C5: empty | R1C6: empty | R1C7: given 99 | R1C8: given 88 | R1C9: given 77 |
R2C1: given 77 | R2C2: empty | R2C3: empty | R2C4: given 44 | R2C5: given 11 | R2C6: given 55 | R2C7: given 33 | R2C8: given 66 | R2C9: given 22 |
R3C1: given 22 | R3C2: given 66 | R3C3: given 33 | R3C4: empty | R3C5: empty | R3C6: empty | R3C7: given 55 | R3C8: given 44 | R3C9: given 11 |
R4C1: given 33 | R4C2: empty | R4C3: given 66 | R4C4: empty | R4C5: given 99 | R4C6: empty | R4C7: given 44 | R4C8: given 77 | R4C9: given 55 |
R5C1: given 55 | R5C2: empty | R5C3: empty | R5C4: empty | R5C5: empty | R5C6: given 66 | R5C7: given 88 | R5C8: given 22 | R5C9: given 99 |
R6C1: empty | R6C2: given 22 | R6C3: empty | R6C4: given 77 | R6C5: given 55 | R6C6: empty | R6C7: given 66 | R6C8: given 11 | R6C9: given 33 |
R7C1: empty | R7C2: empty | R7C3: empty | R7C4: given 55 | R7C5: given 66 | R7C6: given 11 | R7C7: given 22 | R7C8: given 33 | R7C9: given 88 |
R8C1: given 66 | R8C2: given 55 | R8C3: given 11 | R8C4: empty | R8C5: empty | R8C6: empty | R8C7: given 77 | R8C8: given 99 | R8C9: given 44 |
R9C1: target candidates; Before 4, 8, 9 | R9C2: given 33 | R9C3: given 22 | R9C4: empty | R9C5: empty | R9C6: empty | R9C7: given 11 | R9C8: given 55 | R9C9: given 66 |
R9C1 has derived candidates 4, 8, 9. Keep candidates 4, 8, 9; do not place a digit.
- Target cell
- R9C1
- Derived candidates
- 4, 8, 9
- Verified result
- Keep candidates 4, 8, 9; do not place a digit
- Reviewed unit
- Box 7
- Begin with the digits 1 through 9 for one empty cell.
- Remove digits already present in the same row, column, and box.
- Record the complete remaining candidate set.
- After each placement, remove that digit from its peers and look again for singles.
Fixed practice
Choose one empty cell, eliminate digits from 1 through 9, then verify the result separately against its row, column, and box.
Fixed practice · R1C7
R1C1: empty | R1C2: empty | R1C3: empty | R1C4: given 44 | R1C5: empty | R1C6: given 66 | R1C7: target candidates; Before 3, 5, 7 | R1C8: empty | R1C9: given 99 |
R2C1: given 44 | R2C2: empty | R2C3: given 66 | R2C4: given 77 | R2C5: empty | R2C6: empty | R2C7: given 11 | R2C8: given 22 | R2C9: empty |
R3C1: given 77 | R3C2: given 88 | R3C3: empty | R3C4: given 11 | R3C5: empty | R3C6: empty | R3C7: empty | R3C8: empty | R3C9: empty |
R4C1: empty | R4C2: empty | R4C3: empty | R4C4: empty | R4C5: empty | R4C6: given 77 | R4C7: given 88 | R4C8: empty | R4C9: given 11 |
R5C1: empty | R5C2: empty | R5C3: given 77 | R5C4: given 88 | R5C5: given 99 | R5C6: empty | R5C7: given 22 | R5C8: given 33 | R5C9: empty |
R6C1: given 88 | R6C2: empty | R6C3: empty | R6C4: empty | R6C5: given 33 | R6C6: empty | R6C7: empty | R6C8: empty | R6C9: given 77 |
R7C1: empty | R7C2: empty | R7C3: empty | R7C4: given 66 | R7C5: given 77 | R7C6: empty | R7C7: given 99 | R7C8: given 11 | R7C9: given 22 |
R8C1: empty | R8C2: given 77 | R8C3: given 88 | R8C4: empty | R8C5: empty | R8C6: empty | R8C7: empty | R8C8: empty | R8C9: given 55 |
R9C1: given 99 | R9C2: empty | R9C3: empty | R9C4: empty | R9C5: empty | R9C6: empty | R9C7: empty | R9C8: given 77 | R9C9: empty |
R1C7 has derived candidates 3, 5, 7. Keep candidates 3, 5, 7; do not place a digit.
- Target cell
- R1C7
- Derived candidates
- 3, 5, 7
- Verified result
- Keep candidates 3, 5, 7; do not place a digit
- Reviewed unit
- Row 1
Common mistakes
Common mistakes
Treating one possible candidate as the answer without a logical reason that forces it.
Review information
What is verified here
For each shown cell, the verifier recomputes the complete legal candidate set from the visible grid and requires an exact match.
- Fixture ID
- p2-hidden-single-box-01 / p2-hidden-single-row-01
- Engine version
- legen-sudoku-engine/1.3.0
- Verification status
- Exact candidates independently derived
- 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.