Removing the digit before confirming every source-unit position is confined to the intersection.
Sudoku technique · Candidate elimination
Locked Candidates
Locked Candidates removes one digit where the candidate positions in a box and a row or column are confined to their intersection.
Learning order
Topic relationships
These links express the same learning and proof relationships used throughout the library.
- 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: empty; pattern cell; Before 6, 8, 9 | R1C7: given 77 | R1C8: empty | R1C9: empty |
R2C1: given 44 | R2C2: given 55 | R2C3: given 66 | R2C4: given 77 | R2C5: empty | R2C6: empty; pattern cell; Before 2, 8, 9 | R2C7: given 11 | R2C8: empty | R2C9: given 33 |
R3C1: given 77 | R3C2: given 88 | R3C3: given 99 | R3C4: empty | R3C5: empty | R3C6: empty | R3C7: empty | R3C8: empty | R3C9: empty |
R4C1: empty | R4C2: given 33 | R4C3: given 44 | R4C4: given 55 | R4C5: empty | R4C6: given 77 | R4C7: empty | R4C8: empty | R4C9: empty |
R5C1: given 55 | R5C2: given 66 | R5C3: given 77 | R5C4: given 88 | R5C5: given 99 | R5C6: given 11 | R5C7: empty | R5C8: empty | R5C9: given 44 |
R6C1: empty | R6C2: given 99 | R6C3: given 11 | R6C4: empty | R6C5: empty | R6C6: empty | R6C7: given 55 | R6C8: empty | R6C9: given 77 |
R7C1: given 33 | R7C2: given 44 | R7C3: given 55 | R7C4: empty | R7C5: given 77 | R7C6: empty; candidate deletion 9; Before 6, 8, 9; After 6, 8 | R7C7: empty | R7C8: empty | R7C9: given 22 |
R8C1: given 66 | R8C2: given 77 | R8C3: given 88 | R8C4: empty | R8C5: empty | R8C6: empty; candidate deletion 9; Before 2, 4, 5, 9; After 2, 4, 5 | R8C7: empty | R8C8: empty | R8C9: empty |
R9C1: given 99 | R9C2: given 11 | R9C3: given 22 | R9C4: given 33 | R9C5: empty | R9C6: empty | R9C7: given 66 | R9C8: given 77 | R9C9: empty |
Pattern cells: R1C6, R2C6. Remove 9 from R7C6; Remove 9 from R8C6.
- Pattern cells
- R1C6, R2C6
- Pattern digits
- 9
- Source unit
- Box 2
- Target unit
- Column 6
- Variant
- Pointing
Exact candidate changes
- Remove 9 from R7C6Before: 6, 8, 9 → After: 6, 8
- Remove 9 from R8C6Before: 2, 4, 5, 9 → After: 2, 4, 5
- Choose one missing digit and inspect all of its candidate positions in the source unit.
- Confirm that two or three positions are confined to one intersecting unit.
- Keep the digit in the pattern cells because its exact position is not yet known.
- Remove only that digit from the other cells of the intersecting target unit.
Fixed practice
Follow one digit at a time and name both units: the unit that confines it and the unit where candidates can be removed.
Fixed practice · R5C7
R1C1: given 11 | R1C2: given 22 | R1C3: given 33 | R1C4: given 44 | R1C5: given 55 | R1C6: empty | R1C7: given 77 | R1C8: empty | R1C9: empty |
R2C1: given 44 | R2C2: given 55 | R2C3: given 66 | R2C4: given 77 | R2C5: empty | R2C6: empty | R2C7: given 11 | R2C8: empty | R2C9: given 33 |
R3C1: given 77 | R3C2: given 88 | R3C3: given 99 | R3C4: empty | R3C5: empty | R3C6: empty | R3C7: empty | R3C8: empty | R3C9: empty |
R4C1: empty | R4C2: given 33 | R4C3: given 44 | R4C4: given 55 | R4C5: empty | R4C6: given 77 | R4C7: empty; candidate deletion 2; Before 2, 8, 9; After 8, 9 | R4C8: empty; candidate deletion 2; Before 1, 2, 6, 8, 9; After 1, 6, 8, 9 | R4C9: empty |
R5C1: given 55 | R5C2: given 66 | R5C3: given 77 | R5C4: given 88 | R5C5: given 99 | R5C6: given 11 | R5C7: empty; pattern cell; Before 2, 3 | R5C8: empty; pattern cell; Before 2, 3 | R5C9: given 44 |
R6C1: empty | R6C2: given 99 | R6C3: given 11 | R6C4: empty | R6C5: empty | R6C6: empty | R6C7: given 55 | R6C8: empty; candidate deletion 2; Before 2, 6, 8; After 6, 8 | R6C9: given 77 |
R7C1: given 33 | R7C2: given 44 | R7C3: given 55 | R7C4: empty | R7C5: given 77 | R7C6: empty | R7C7: empty | R7C8: empty | R7C9: given 22 |
R8C1: given 66 | R8C2: given 77 | R8C3: given 88 | R8C4: empty | R8C5: empty | R8C6: empty | R8C7: empty | R8C8: empty | R8C9: empty |
R9C1: given 99 | R9C2: given 11 | R9C3: given 22 | R9C4: given 33 | R9C5: empty | R9C6: empty | R9C7: given 66 | R9C8: given 77 | R9C9: empty |
Pattern cells: R5C7, R5C8. Remove 2 from R4C7; Remove 2 from R4C8; Remove 2 from R6C8.
- Pattern cells
- R5C7, R5C8
- Pattern digits
- 2
- Source unit
- Row 5
- Target unit
- Box 6
- Variant
- Claiming
Exact candidate changes
- Remove 2 from R4C7Before: 2, 8, 9 → After: 8, 9
- Remove 2 from R4C8Before: 1, 2, 6, 8, 9 → After: 1, 6, 8, 9
- Remove 2 from R6C8Before: 2, 6, 8 → After: 6, 8
Common mistakes
Common mistakes
Deleting the candidate from a pattern cell or from a cell outside the intersecting target unit.
Review information
What is verified here
The independent verifier recomputes the canonical pointing or claiming pattern, its source and target units, and every exact deletion before the state can change.
- Fixture ID
- p3b-locked-pointing-01 / p3b-locked-claiming-01
- Engine version
- legen-sudoku-engine/1.3.0
- Verification status
- Independent candidate-elimination 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.