Sudoku Locked Candidates is one of the first intermediate techniques most Sudoku solvers learn after mastering Naked Singles and Hidden Singles. It exploits the interaction between boxes and lines (rows or columns) to eliminate candidates.
The idea is simple: if a candidate digit inside a box can only appear in one row or column, that digit is “locked” into that line within the box. You can then remove the candidate from the rest of that line (Pointing) or from the rest of the box (Claiming).
In this guide you will learn both types, walk through a real puzzle example, and discover practical tips for spotting Locked Candidates quickly.
✅ What Are Locked Candidates?
A Locked Candidate occurs whenever a candidate digit’s possible positions inside a box are restricted to a single row or column. Because that digit must go in one of those cells, it cannot appear elsewhere in that row or column (or elsewhere in the box, depending on the perspective).
There are two complementary forms:
- Type 1 – Pointing: Start from the box; eliminate along the line.
- Type 2 – Claiming: Start from the line; eliminate inside the box.
Both forms use the same underlying logic — box–line interaction — but they look at the pattern from opposite directions.
Locked Candidates is the umbrella term. You may also hear Pointing Pairs / Pointing Triples for Type 1 and Box/Line Reduction or Claiming for Type 2. They are all the same family of techniques.
👉 Type 1: Pointing
If a candidate digit within a box is confined to one row (or one column), that digit can be eliminated from all cells in that row (or column) outside the box.
Why it works: The digit must appear once in the box. Since it can only go in cells along a single line inside the box, it will definitely occupy that line. No other cell in that same line (outside the box) can hold the digit.
Example pattern: In Box 6, candidate 5 appears only in R6C7 and R6C9 — both in Row 6. Candidate 5 “points” along Row 6, so you can eliminate 5 from any other cell in Row 6 that is outside Box 6.
When exactly two cells hold the locked candidate, this is also called a Pointing Pair. With three cells, it is a Pointing Triple.
📦 Type 2: Claiming (Box/Line Reduction)
If a candidate digit within a row (or column) is confined to one box, that digit can be eliminated from all other cells in that box outside the row (or column).
Why it works: The digit must appear once in the row. Since all its possible positions in that row fall within a single box, the digit is claimed by that box through the line. No other cell in the box (in a different row) can hold it.
Example pattern: In Row 3, candidate 8 appears only in C1 and C3 — both in Box 1. Row 3 “claims” digit 8 for Box 1, so 8 can be removed from all other cells in Box 1 that are not in Row 3.
This form is also known as Box/Line Reduction.
🔎 Step-by-Step Example
Let’s walk through a real Locked Candidate (Pointing) example for digit 5 in Box 6.
Step 1: Scan Box 6
Look at Box 6 (R4–R6, C7–C9). Where can digit 5 go? After pencil-marking, candidate 5 appears only in two cells:
- R6C7 = {3,4,5,7}
- R6C9 = {2,4,5,7}
No other cell in Box 6 has candidate 5.
Step 2: Identify the Lock
Both cells are in Row 6. The digit 5 in Box 6 is locked into Row 6 — this is a Pointing Pair on digit 5.
Step 3: Eliminate Along the Line
Since digit 5 in Row 6 must come from Box 6, remove it from every other cell in Row 6 outside the box:
- R6C4 — {1,4,
5,7} → {1,4,7} - R6C6 — {4,
5,6,7,9} → {4,6,7,9}
Step 4: Result
Two cells lose candidate 5, narrowing their possibilities and potentially enabling further techniques like Naked Pairs or Hidden Singles in the affected row.
Pointing (Type 1): Candidate in a box is confined to one line → eliminate from the rest of that line outside the box.
Claiming (Type 2): Candidate in a line is confined to one box → eliminate from the rest of that box outside the line.
Both exploit box–line interaction.
⚖️ Pointing vs. Claiming: Quick Comparison
| Aspect | Pointing (Type 1) | Claiming (Type 2) |
|---|---|---|
| Start from | Box | Row or Column |
| Observation | Candidate confined to one line within the box | Candidate confined to one box within the line |
| Eliminate from | Rest of the line (outside box) | Rest of the box (outside line) |
| Also called | Pointing Pair / Pointing Triple | Box/Line Reduction, Claiming |
| Cells involved | 2 or 3 (Pair or Triple) | 2 or 3 (Pair or Triple) |
🕵️ How to Spot Locked Candidates
For Pointing (Type 1):
1. Pick a box.
2. For each candidate digit 1–9, check which cells in the box contain it.
3. If all those cells fall in a single row or column → eliminate along that line outside the box.
For Claiming (Type 2):
1. Pick a row or column.
2. For each candidate digit 1–9, check which cells in the line contain it.
3. If all those cells fall in a single box → eliminate from the rest of that box outside the line.
Shortcut: Focus on digits that have few remaining positions in a box or line. If a digit has only 2–3 candidate cells, there is a higher chance they align in a single row, column, or box.
⚠️ Common Mistakes
1. Eliminating in the Wrong Direction
With Pointing you eliminate along the line outside the box. With Claiming you eliminate inside the box outside the line. Mixing these up leads to incorrect eliminations.
2. Forgetting Already-Solved Cells
If a cell in the box is already solved for the digit in question, that digit is not a candidate elsewhere in the box, row, or column. Make sure your pencil marks are up to date before scanning.
3. Missing the Triple
Locked Candidates works with three cells too, not just pairs. If three cells in a box on the same row hold a candidate, the logic is identical — eliminate from the rest of the row outside the box.
4. Overlooking Claiming Opportunities
Many solvers learn Pointing first and forget about Claiming. Scan lines as well as boxes to catch both forms.
📅 When to Look for Locked Candidates
- Basic: Naked Singles, Hidden Singles, Full House.
- Early intermediate: Locked Candidates (Pointing & Claiming).
- Intermediate: Naked Pairs, Hidden Pairs, Naked Triples, Hidden Triples.
- Advanced: X-Wing, Skyscraper, Simple Colouring, Unique Rectangles.
- Expert: Swordfish, Jellyfish, XY-Wing, W-Wing.
Locked Candidates sit just above basic techniques. Apply them before Naked Pairs and other intermediate methods. They are fast to check and frequently productive.
Puzzles requiring Locked Candidates are typically rated Medium and above. Try our medium puzzles or hard puzzles for practice.
🚀 Beyond Locked Candidates
| Technique | What It Adds | Complexity |
|---|---|---|
| Locked Candidates | Box–line interaction for a single digit | Early intermediate |
| Naked Pairs | Two cells sharing two candidates → eliminate from peers | Intermediate |
| X-Wing | Two rows × two columns for one digit → column eliminations | Advanced |
| Swordfish | Three rows × three columns for one digit | Expert |
| Pointing Pairs | Specific case of Locked Candidates Type 1 | Early intermediate |
| Box/Line Reduction | Specific case of Locked Candidates Type 2 | Early intermediate |
Locked Candidates prepare you for more complex single-digit techniques. X-Wing extends the line-based logic to two rows and columns, while Swordfish expands it to three. Meanwhile, Naked Pairs and Hidden Pairs deal with multi-digit interactions within a house.
🎯 Practice Locked Candidates
Pointing Pairs Guide
A focused guide on Locked Candidates Type 1 with additional examples.
▶ Read Pointing Pairs GuideBox/Line Reduction Guide
A focused guide on Locked Candidates Type 2 with additional examples.
▶ Read Box/Line Reduction GuideFrequently Asked Questions
Locked Candidates is an elimination technique based on box–line interactions. When a candidate in a box is confined to one row or column, it can be eliminated from that line outside the box (Pointing) or from the rest of the box (Claiming).
Pointing starts from a box: the candidate is confined to one line within the box, so eliminate from that line outside the box. Claiming starts from a line: the candidate is confined to one box within the line, so eliminate from the rest of that box.
Pointing Pairs are a specific case of Locked Candidates Type 1 with exactly two cells. Locked Candidates is the broader umbrella covering Pointing (Type 1) with pairs or triples, and Claiming (Type 2, also called Box/Line Reduction).
After exhausting Naked Singles and Hidden Singles. Locked Candidates are an early intermediate technique — easier than Naked Pairs or X-Wing — and one of the first elimination strategies solvers learn after the basics.
Locked Candidates alone rarely solve hard puzzles, but they are a crucial stepping stone. They reduce candidates so that advanced techniques like X-Wing and Swordfish can be applied. Even expert-level puzzles often have Locked Candidates opportunities early on.