You’re staring at a Sudoku grid that seems completely stuck. Every row, column, and box looks like a jumble of possibilities, and the simple techniques you relied on before — naked singles, hidden singles — just aren’t cutting it anymore. Before you reach for the answer key or start guessing, there’s a powerful intermediate technique you should try: Locked Candidates. This method lets you eliminate pencil marks you never thought you could remove, often breaking open a puzzle that felt completely frozen. Once you understand it, you’ll wonder how you ever managed without it.
What Are Locked Candidates?
In Sudoku, the term locked candidates refers to a situation where a particular digit’s possible positions within a box are restricted to a single row or column — or where a digit’s possible positions within a row or column are all confined to a single box. When either of these conditions is true, you can use that confinement to eliminate the same digit from other cells you might not have considered.
The logic is straightforward: every digit from 1 to 9 must appear exactly once in every row, every column, and every 3×3 box. If a digit can only go in certain places, and all those places share a common region, then the digit is effectively “locked” into that shared region. Anything outside that region that intersects it can safely have that digit removed from its candidate list.
There are two distinct types of Locked Candidates, and each works in a slightly different direction:
- Type 1 — Pointing Pairs (or Pointing Triples): A digit’s candidates within a box all fall in the same row or column. That digit can be eliminated from the rest of that row or column outside the box.
- Type 2 — Claiming (or Box-Line Reduction): A digit’s candidates within a row or column all fall inside the same box. That digit can be eliminated from the rest of that box outside the row or column.
Both types are governed by the same underlying logic; only the direction of the elimination changes. Let’s explore each one with a concrete example.
Type 1: Pointing Pairs and Pointing Triples
Imagine you are working with the top-left 3×3 box of a puzzle. After scanning for existing digits and ruling out candidates, you determine that the digit 7 can only appear in two cells within that box: R1C2 and R1C3 (Row 1, Columns 2 and 3). Both of these candidate cells are inside the top-left box, and both lie in Row 1.
Here is the key insight: because the digit 7 must go somewhere in that top-left box, and its only options are in Row 1, it will end up in Row 1. That means no other cell in Row 1 — outside of that box — can contain a 7. You can safely remove 7 as a candidate from R1C4, R1C5, R1C6, R1C7, R1C8, and R1C9 (all of Row 1 that lies outside the top-left box).
This is called a Pointing Pair because two cells are “pointing” at the row (or column) that benefits from the elimination. If three cells in the box all shared the same row or column, it would be called a Pointing Triple — the logic is identical, just with one more cell involved.
Let’s make this even more concrete. Suppose the grid looks like this in the top-left box:
- R1C1: contains a given digit (say, 3)
- R1C2: candidates include 2, 7
- R1C3: candidates include 5, 7
- R2C1: candidates include 2, 4
- R2C2: contains a given digit (say, 8)
- R2C3: candidates include 1, 4, 6
- R3C1: candidates include 1, 6
- R3C2: candidates include 4, 9
- R3C3: candidates include 1, 6, 9
Scanning for the digit 7 in this box: it can only appear at R1C2 or R1C3. Both are in Row 1. Therefore, any cell in Row 1 that is outside this box — R1C4 through R1C9 — cannot be 7, no matter what other candidates they might have. You can erase 7 from all of them. That might seem like a small win, but in a hard puzzle, eliminating even one candidate can trigger a cascade of new deductions.
Type 2: Claiming (Box-Line Reduction)
The second type works in the opposite direction. Instead of starting from a box and looking outward, you start from a row or column and look inward at a box.
Suppose you are working on Row 4. After filling in known digits and eliminating candidates based on the existing numbers in Row 4’s intersecting columns and boxes, you find that the digit 5 can only appear in three cells in Row 4: R4C1, R4C2, and R4C3. Notice that all three of these cells fall within the middle-left 3×3 box (let’s say it occupies rows 4–6, columns 1–3).
The logic: since 5 must go somewhere in Row 4, and all its Row 4 options are inside the middle-left box, the digit 5 will land in that box via Row 4. That means no other cell in the middle-left box — R5C1, R5C2, R5C3, R6C1, R6C2, and R6C3 — can contain a 5. You eliminate 5 from all of those cells.
This technique is sometimes called Box-Line Reduction or simply “claiming,” because the row (or column) is “claiming” a digit for a particular box, allowing you to reduce candidates elsewhere in that box.
Both Type 1 and Type 2 are logically clean — no guessing, no trial and error. They follow directly from the fundamental rules of Sudoku, which is what makes them so satisfying to apply.
How to Spot Locked Candidates in a Real Puzzle
Knowing the theory is one thing; finding locked candidates in a real puzzle requires a systematic approach. Here is a step-by-step method you can follow:
- Maintain a full candidate list. You cannot spot locked candidates without knowing all possible values for each empty cell. Use pencil marks (either on paper or in a digital puzzle app) to record every digit that could legally go in each cell based on existing values in the same row, column, and box.
- Scan box by box for Type 1. For each 3×3 box, look at each digit from 1 to 9. Note where that digit can go within the box. If all its positions fall in the same row or column, you have a Pointing Pair or Triple. Eliminate that digit from the rest of that row or column.
- Scan row by row and column by column for Type 2. For each row and column, look at where each digit can go. If all positions for a digit fall within the same box, you have a Claiming situation. Eliminate that digit from the rest of that box.
- Repeat after every elimination. Removing a candidate can create new locked candidates that weren’t visible before. After each successful elimination, scan again from the beginning.
- Combine with other techniques. Locked Candidates work especially well alongside naked pairs, hidden pairs, and X-Wings. After applying locked candidates, you may suddenly find naked singles or hidden singles that were previously hidden.
One common mistake beginners make is skipping the candidate maintenance step. If you haven’t carefully tracked all possible values, you might miss a locked candidate or, worse, incorrectly apply the technique. Always keep your pencil marks up to date before scanning for this pattern.
Why Locked Candidates Matter for Hard Puzzles
Locked Candidates sits right at the boundary between basic and advanced Sudoku strategy. Most easy and medium puzzles can be solved with naked singles and hidden singles alone. But as soon as you step up to hard or expert-level Sudoku — including puzzles rated as “diabolical” or those with minimal given digits — the simpler techniques hit a wall.
This is where Locked Candidates becomes indispensable. In a typical hard puzzle, there will be several moments where no naked single or hidden single is available. Applying the Pointing Pair technique might not immediately place a digit, but it reduces the candidate pool, which then reveals a hidden single that was buried. This chained reaction of technique leading to technique is the hallmark of solving harder Sudoku puzzles correctly.
Additionally, understanding Locked Candidates builds the mental framework needed for even more advanced techniques. Concepts like X-Wings, Swordfish, and XY-Wings all depend on the same foundational idea: when candidates are restricted to certain intersecting regions, eliminations become possible. Mastering Locked Candidates is essentially mastering the underlying logic that powers the entire family of elimination-based strategies.
Regular practice with this technique also sharpens your overall puzzle vision. Players who learn to spot pointing pairs quickly develop a more structured scanning habit, which speeds up their solving time across all difficulty levels — not just on the puzzles where Locked Candidates are strictly necessary.
Common Mistakes and How to Avoid Them
Even experienced solvers occasionally slip up when applying Locked Candidates. Here are the most frequent errors and how to prevent them:
- Eliminating from the wrong region: In a Pointing Pair, you eliminate candidates from the row or column outside the box, not from the box itself. Double-check which region you are removing candidates from before you erase.
- Forgetting to recheck after eliminations: A single locked candidate application can open up multiple new moves. Always rescan the puzzle after making any elimination.
- Applying the technique with incomplete candidate lists: If you haven’t correctly computed all candidates for every cell, you might falsely identify a locked candidate. Verify each cell’s candidates against its row, column, and box before committing.
- Confusing Type 1 and Type 2: Remember — Type 1 starts in a box and eliminates outward; Type 2 starts in a line and eliminates within a box. Mixing them up leads to wrong eliminations.
Key Takeaways
The Locked Candidates technique is one of the most important tools in any intermediate Sudoku solver’s toolkit. Here’s a quick summary of everything covered:
- Locked Candidates exploit the intersection between rows/columns and 3×3 boxes to eliminate candidates that cannot be correct.
- Type 1 (Pointing Pairs/Triples): When a digit’s candidates in a box all fall on the same row or column, eliminate that digit from the rest of that row or column.
- Type 2 (Claiming/Box-Line Reduction): When a digit’s candidates in a row or column all fall within the same box, eliminate that digit from the rest of that box.
- Always maintain accurate, complete pencil marks before scanning for this technique.
- Locked Candidates often trigger a cascade — eliminating one candidate can reveal naked singles or hidden singles elsewhere.
- Mastering this technique builds the logical foundation for harder strategies like X-Wings and Swordfish.
Every Sudoku expert started out facing puzzles that felt impossibly stuck, just like you might today. The difference between someone who gives up and someone who breaks through is often a single technique applied at the right moment. Locked Candidates is frequently that technique. Load up a hard puzzle on playsudoku.org, switch on pencil marks, and start scanning — your breakthrough moment might be just one pointing pair away.