Interactive Sudoku Technique
Locked Candidates: Pointing and Claiming
Locked Candidates use the three-cell intersection between a box and a row or column. The same confinement works in two directions: a box can point into a line, or a line can claim a box.
The Relationship
Follow the Direction of the Proof
Box Line
Every position for one digit inside a box lies on the same row or column. Eliminate that digit farther along the line outside the box.
Line Box
Every position for one digit on a row or column lies inside the same box. Eliminate that digit elsewhere inside the box.
Pointing pushes outward. Claiming cleans inward.
Why It Works
The Digit Must Occupy the Shared Intersection
A row, column, and box must each contain a digit exactly once. When every remaining position for that digit in one house lies inside its intersection with another house, one of those shared cells must contain the digit. The rest of the second house can no longer contain it.
Two source cells are commonly called a Pointing Pair or Claiming Pair. Three source cells form a triple. The logic does not depend on knowing which source cell will eventually receive the digit.
Interactive Example 1 of 2
Find the Eliminations
This board isolates one digit's candidate map. Shaded cells form the source pattern. Select every candidate eliminated by the lock.
A SudokuParadise Example
Two Independent Proofs, the Same Elimination
Pointing and Claiming can coexist as separate patterns and converge on the same victims. The technique name identifies the source of the proof—not necessarily a unique elimination.
-
Claiming: Row 2's only candidate 3s are
r2c2andr2c3, both inside Box 1. Row 2 claims digit 3 in Box 1. -
Pointing: Box 2's only candidate 3s are
r3c5andr3c6, both on Row 3. Box 2 points digit 3 across Row 3. -
Shared result: Either proof eliminates 3 from
r3c2andr3c3.
Compare the Directions
What Disqualifies One May Be the Victim of the Other
| Question | Pointing | Claiming |
|---|---|---|
| Where do you begin? | Inside one box | Along one row or column |
| What must be confined? | All box candidates must share one line | All line candidates must share one box |
| What disqualifies it? | An extra candidate inside the box but off the line | An extra candidate on the line but outside the box |
| Where are the victims? | On the line outside the box | Inside the box but off the line |
Interactive Check
Locked Candidate or Near Miss?
Choose the best classification. Each description supplies all the information needed; do not assume missing candidates have already been eliminated.
1. The Extra Box Candidate
In Box 1, candidate 6 appears at r2c1,
r2c3, and r3c2. Row 2 also contains a
6 at r2c8.
Choose a classification.
The Box 1 candidates span two rows, so they do not point. Row 2 also has a candidate outside Box 1, so it does not claim.
2. The Line Claims the Box
Row 5 contains candidate 7 only at r5c4 and
r5c6. Both cells belong to Box 5.
Choose a classification.
Every 7 on Row 5 is confined to Box 5, so Row 5 claims digit 7 inside that box.
3. The Diagonal Pair
The only candidate 4s in Box 7 are r7c1 and
r8c2. They occupy diagonal positions.
Choose a classification.
This is a two-location box conjugate, useful for Snyder notes, but the cells share neither a row nor a column and cannot point.
4. The One-Cell Boundary
Column 8 has only one remaining position for candidate 2:
r6c8.
Choose a classification.
With only one possible cell left in the column, place 2 at r6c8. A placement takes priority over a Locked Candidate elimination.
The Boundary Case
One Candidate Is a Hidden Single
Locked Candidates normally preserve uncertainty between two or three cells. If the source house has only one possible location, the uncertainty has disappeared. Place the digit as a Hidden Single instead of describing the position as Pointing or Claiming.
- One location remaining in a box: Hidden Single in the box.
- One location remaining on a row: Hidden Single in the row.
- One location remaining on a column: Hidden Single in the column.
Common Questions
Locked Candidates Clarified
Do Pointing and Claiming require exactly two cells?
No. Two aligned source cells form a pair, and three form a triple. What matters is that every candidate for the digit in the source house lies within the same box-line intersection.
Can Pointing and Claiming eliminate the same candidates?
Yes. Separate Pointing and Claiming patterns can converge on the same victim cells, as shown in the SudokuParadise illustration on this page.
Does a pair of candidates anywhere in a box point?
No. Every candidate for the selected digit in that box must share one row or one column. A diagonal pair does not point.
Can a candidate outside the box be part of Pointing?
It is not part of the source pattern, but it may be a victim. For Claiming, that same outside candidate would disqualify the line from being confined to the box.
Continue Practicing
Look for Locked Candidates in a Complete Puzzle
Generate a fresh Sudoku, enter one from another source, or practice candidate discipline with the interactive Snyder Notation lesson.