Intermediate Sudoku Strategy
Locked Candidates Sudoku: Pointing Pairs and Box-Line Reduction
Learn how candidates become locked between a 3×3 box and a row or column. Use pointing pairs, pointing triples, and box-line reduction to remove candidates without guessing.
One Candidate, One Line
When every possible position for one number is restricted to a single row or column, that number is locked into that line.
Start With a Box
Check whether every position for one candidate inside a box lies in the same row or column.
Follow the Line
The candidate is locked into that line, creating eliminations outside the original box.
Remove Safely
Remove only the locked candidate from cells directly affected by the pattern.
What Are Locked Candidates in Sudoku?
Locked Candidates are candidate-elimination patterns created by the relationship between a 3×3 box and a row or column.
If every possible position for one number inside a box lies in the same row, that number must appear somewhere in that part of the row. It therefore cannot appear elsewhere in the same row outside the box.
The same logic works in reverse. If every possible position for one number in a row lies inside the same box, that candidate can be removed from the other cells in that box.
The Two Types of Locked Candidates
Pointing Pair or Pointing Triple
Inside one box, all positions for a candidate lie in the same row or column. Remove that candidate from the rest of the line outside the box.
Claiming Pair or Box-Line Reduction
Inside one row or column, all positions for a candidate lie in the same box. Remove that candidate from the other cells in the box.
How a Pointing Pair Works
Choose One 3×3 Box
Look at one box and choose one missing candidate number.
Find Every Position for the Candidate
Confirm that the candidate appears in exactly two cells inside the box.
Check Whether the Cells Share One Line
The two cells must lie in the same row or the same column.
Remove the Candidate Outside the Box
Remove that candidate from other cells in the shared row or column outside the box.
Pointing Pair Example
Suppose candidate 6 appears only in R2C1 and R2C3 inside the top-left box. Both possible positions lie in row 2.
Candidate Positions
Because the 6 for the top-left box must be placed in row 2, candidate 6 can be removed from every other unsolved cell in row 2 outside that box.
How a Pointing Triple Works
A Pointing Triple follows the same rule as a Pointing Pair, but the candidate appears in three cells instead of two.
Pointing Triple Example
If candidate 4 appears only in three cells of the centre box and all three cells lie in column 5, candidate 4 can be removed from every other cell in column 5 outside the centre box.
The number of positions does not matter as much as the alignment. Every candidate position inside the box must lie on the same row or column.
How Box-Line Reduction Works
Choose One Row or Column
Look at one line and select one missing candidate.
Find Every Position for That Candidate
Confirm that all possible positions in the line are located inside the same 3×3 box.
Claim the Candidate for the Line
That candidate must appear somewhere in the shared part of the line and box.
Remove the Candidate Elsewhere in the Box
Delete that candidate from other cells in the same box that do not belong to the selected line.
Box-Line Reduction Example
Suppose candidate 8 can appear in row 7 only at R7C4 and R7C6. Both cells belong to the bottom-middle box.
What You Can Eliminate
Because row 7 must place its 8 inside the bottom-middle box, candidate 8 can be removed from every other cell in that box outside row 7.
This is the reverse direction of a Pointing Pair. Pointing starts with the box and eliminates from the line. Box-line reduction starts with the line and eliminates from the box.
Pointing vs. Claiming
Why Locked Candidates Work
Every number must appear exactly once in each row, column, and 3×3 box.
If a box can place candidate 5 only in one row, then one of those positions must contain 5. That guarantees the same row cannot place another 5 outside the box.
The Key Relationship
A candidate restricted to the intersection of one box and one line controls both units. The restriction in one unit creates eliminations in the other.
When Should You Look for Locked Candidates?
After Singles Stop
Locked Candidates are one of the first techniques to check after basic scanning and singles.
When a Box Has Few Candidate Positions
A candidate appearing only two or three times inside a box is worth checking for alignment.
In Medium and Hard Sudoku
The technique commonly appears in Medium and Hard puzzles and can also support Expert solving.
Before Complex Patterns
Check Locked Candidates before X-Wing, Swordfish, XY-Wing, or chains.
When Is It Not a Locked Candidate?
❌ A Candidate Appears in More Than One Line
If candidate positions inside the box span different rows and columns, the candidate is not locked to one line.
❌ The Line Uses More Than One Box
For box-line reduction, every candidate position in the chosen line must lie inside the same box.
❌ Candidate Notes Are Incomplete
A missing candidate can make a false pointing or claiming pattern appear.
❌ The Elimination Is Inside the Wrong Unit
Pointing eliminates from the line outside the box. Claiming eliminates from the box outside the line.
Common Locked Candidates Mistakes
❌ Removing the Candidate From Pattern Cells
Keep the candidate in the cells that create the lock. Remove it only from affected cells outside the intersection.
❌ Confusing Pointing and Claiming
Remember: pointing starts in a box; claiming starts in a row or column.
❌ Assuming Exactly Two Cells Are Required
A pointing pattern can use two or three cells. The essential rule is that every position lies on one line.
❌ Expecting an Immediate Placement
Locked Candidates usually remove notes. Rescan afterward to find the next direct placement.
Locked Candidates vs. Naked Pair
Continue with our Naked Pair Sudoku guide .
Locked Candidates vs. Hidden Pair
How to Practise Locked Candidates
1. Complete Candidate Notes
Accurate candidate positions are essential for finding valid locks.
2. Scan Box by Box
For each number, check whether all positions inside a box align in one row or column.
3. Reverse the Search
Then scan rows and columns to see whether all candidate positions lie inside one box.
4. Rescan After Every Elimination
A single removed candidate may reveal a single, Naked Pair, Hidden Pair, or another lock.
Locked Candidates Practice Checklist
- 1. Are the candidate notes complete and current?
- 2. Are all candidate positions inside the box on the same row or column?
- 3. For claiming, are all positions in the line inside one box?
- 4. Are you eliminating only the same candidate?
- 5. Are you removing it from the correct side of the box-line relationship?
- 6. Did you keep the candidate in the cells that form the lock?
- 7. Did you rescan for new singles afterward?
Related Sudoku Learning Pages
🧠 Advanced Sudoku Techniques
Explore subsets, X-Wing, Swordfish, XY-Wing, coloring, unique rectangles, and chains.
👯 Naked Pair Sudoku
Learn how matching bivalue cells remove candidates from the rest of a unit.
🕵️ Hidden Pair Sudoku
Find two candidates hidden inside the same two cells.
✖️ X-Wing Sudoku
Move on to a four-corner candidate pattern across two rows and columns.
Frequently Asked Questions
What are Locked Candidates in Sudoku?
Locked Candidates occur when a candidate is restricted to the intersection of one 3×3 box and one row or column, allowing eliminations in the connected unit.
What is a pointing pair?
A pointing pair occurs when a candidate appears in two cells inside one box and both cells lie in the same row or column.
What is box-line reduction?
Box-line reduction occurs when all positions for a candidate in one row or column lie inside the same box.
Are Locked Candidates advanced?
They are generally considered intermediate and are a good technique to learn before X-Wing and other advanced patterns.
Do Locked Candidates place a number directly?
Usually not. They remove candidates and often reveal the next single or solving pattern.
Practise Locked Candidates
Open a Medium or Hard puzzle, turn on Notes, and scan every 3×3 box for candidates aligned in one row or column.