If you have ever stared at a hard Sudoku puzzle feeling completely stuck after using basic techniques like naked singles and hidden singles, you are not alone. Most puzzles rated medium, hard, or expert require at least one more tool in your solving kit. One of the most powerful — and most satisfying — intermediate techniques is called Locked Candidates. Once you understand it, you will start spotting opportunities that were previously invisible, and you will find yourself breaking through walls that once stopped you cold.
What Are Locked Candidates?
Locked Candidates is a technique based on a straightforward but important logical principle: when a particular candidate digit can only appear in one row or one column within a specific 3×3 box, that digit must eventually be placed somewhere in that row or column. As a result, you can safely eliminate that digit as a candidate from every other cell in the same row or column that lies outside the box.
In other words, the candidate is “locked” to the intersection of a box and a line (row or column). Because it has to go somewhere inside that box along that specific line, it cannot also be a valid option anywhere else along the same line.
There are two main variations of this technique:
- Type 1 (Pointing Pairs or Pointing Triples): A candidate appears only within one row or column inside a box. You eliminate it from the rest of that row or column outside the box.
- Type 2 (Box-Line Reduction): A candidate appears only within one box along a particular row or column. You eliminate it from the rest of that box.
Both variations rely on the same core logic but attack the puzzle from slightly different directions. Together they cover a surprising number of situations that arise in intermediate and hard Sudoku puzzles.
Locked Candidates Type 1: Pointing Pairs and Pointing Triples
Let’s walk through a concrete example of Type 1, sometimes called a Pointing Pair or Pointing Triple.
Imagine you are working on a puzzle and you focus on the top-right 3×3 box (columns 7, 8, and 9, rows 1, 2, and 3). You mark every possible candidate for every empty cell in that box. After careful pencil-marking, you notice that the digit 5 can only appear in two cells, and both of those cells are in row 2 — specifically in column 7 and column 8.
Here is the key insight: because the digit 5 must go somewhere in that top-right box, and both possible positions are in row 2, the 5 must be placed in row 2 somewhere in that box. That means no other cell in row 2 — in the top-middle box (columns 4, 5, 6) or the top-left box (columns 1, 2, 3) — can contain a 5.
You can now erase the digit 5 from every other cell in row 2 outside the top-right box. Even if you do not know exactly which of the two cells (column 7 or column 8) will hold the 5, you have still made real progress. Other cells in that row that previously listed 5 as a possibility are now simplified, and this often triggers a cascade of new deductions.
The same logic applies when three cells in a box all share the same row or column for a particular candidate — this is called a Pointing Triple. The principle is identical: the candidate is locked to that row or column within the box, so it can be removed from the rest of the row or column elsewhere in the grid.
When searching for Pointing Pairs and Triples, it helps to work box by box rather than scanning randomly. Pick a box, pick a digit, and ask: “In how many rows and columns does this digit appear as a candidate?” If the answer is just one row or one column, you have found a Pointing Pair or Triple.
Locked Candidates Type 2: Box-Line Reduction
Type 2 works in the opposite direction. Instead of starting with a box and looking outward, you start with a row or column and look inward at a box.
Here is a worked example of Box-Line Reduction. Suppose you are studying row 6. You mark all candidates in row 6 and notice that the digit 3 appears as a candidate in three cells of that row: one in the left box (columns 1–3) and two in the middle box (columns 4–6). Actually, let’s sharpen the scenario: the digit 3 appears as a candidate in row 6 only within the cells belonging to the middle box — say in column 4 and column 5.
Because the only places in row 6 where a 3 can go are column 4 and column 5, and both of those cells belong to the middle box of row 6, the digit 3 must eventually be placed in the middle box in row 6. This means the digit 3 cannot appear anywhere else in the middle box — not in rows 4 or 5 within that box.
You can now eliminate 3 as a candidate from all other cells in the middle 3×3 box that are not in row 6. This is the Box-Line Reduction: the candidate is confined to one box along a specific row (or column), so it is eliminated from the rest of that box.
Type 2 is slightly less intuitive for beginners, but once you train your eye to look at rows and columns as potential “reducers” of box candidates, it becomes a natural part of your scanning routine.
Why Locked Candidates Matter in Real Puzzles
The Locked Candidates technique sits at a crucial level in the hierarchy of Sudoku solving strategies. It is more powerful than basic singles but does not require the complex pattern recognition of techniques like X-Wings, Swordfish, or XY-Chains. This makes it an essential bridge strategy — the skill that separates casual solvers from consistent puzzle-finishers.
Here is why this technique appears so frequently in hard puzzles:
- Clears pencil marks efficiently: Removing candidates from multiple cells in one move accelerates your progress dramatically.
- Triggers new singles: When you eliminate a candidate from several cells, other cells in the same row, column, or box may suddenly have only one candidate left — revealing a naked single or hidden single you could not see before.
- Works alongside other techniques: Locked Candidates often combines naturally with naked pairs, hidden pairs, and other elimination strategies, giving you a layered approach to tough puzzles.
- Reduces guessing: One of the most common mistakes intermediate solvers make is resorting to trial and error (also called “bifurcation”) before exhausting logical techniques. Locked Candidates gives you a clean, logical path forward.
Professional and competitive Sudoku solvers regard Locked Candidates as a foundational intermediate technique. Many puzzles published in newspapers and online platforms that are rated “hard” can be solved entirely with a combination of basic singles and Locked Candidates — no advanced strategies required. Mastering this technique alone significantly expands the range of puzzles you can confidently solve from start to finish.
How to Practice Finding Locked Candidates
Like any skill, finding Locked Candidates gets easier with deliberate practice. Here are some strategies to sharpen your eye:
- Always use pencil marks (candidate notation): You simply cannot spot Locked Candidates reliably without first filling in all possible candidates for every empty cell. This is non-negotiable for intermediate and hard puzzles.
- Scan box by box for Type 1: Take each 3×3 box and check every digit from 1 to 9. For each digit, note which rows and columns the candidate cells occupy. If all candidate cells share one row or column, you have a Pointing Pair or Triple.
- Scan row by row and column by column for Type 2: For each row (or column), check every digit. If all cells in the row where a candidate appears belong to the same box, you have a Box-Line Reduction.
- Work systematically, not randomly: Random scanning wastes time and causes you to miss patterns. Develop a consistent sequence — for example, always start with box 1 and work across and down.
- Use solver tools for verification: When learning, it helps to check your work. Many free Sudoku apps and websites show you which technique applies to a given puzzle position, so you can verify that you identified the Locked Candidates correctly.
- Revisit after each move: After any elimination, go back and check whether the change creates new Locked Candidates opportunities. The grid is constantly evolving as you fill in digits.
Beginners often find Type 1 (Pointing Pairs) easier to spot first, so it is a good idea to start there. Practice on puzzles rated “medium” before moving to “hard,” and you will find that you start seeing Pointing Pairs almost automatically after a few sessions.
Locked Candidates vs. Other Intermediate Techniques
It is worth understanding where Locked Candidates fit in the broader landscape of Sudoku strategies. Techniques are generally ordered by complexity:
- Basic: Naked Single, Hidden Single
- Intermediate: Locked Candidates (Types 1 and 2), Naked Pairs, Hidden Pairs, Naked Triples
- Advanced: X-Wing, Swordfish, XY-Wing, Skyscraper
- Expert: Unique Rectangles, Forcing Chains, Alternating Inference Chains (AICs)
Locked Candidates are among the easiest intermediate techniques to learn and apply. Unlike X-Wings, which require you to scan across two separate rows or columns simultaneously, Locked Candidates only ask you to look at the interaction between one box and one line at a time. This focused scope makes them far more accessible while still delivering meaningful eliminations.
If you find yourself using Naked Pairs and Hidden Pairs confidently, adding Locked Candidates to your repertoire is a natural and highly rewarding next step.
Key Takeaways
- Locked Candidates is an intermediate Sudoku strategy based on the interaction between a 3×3 box and a row or column.
- Type 1 (Pointing Pairs/Triples): When a candidate in a box is confined to one row or column, eliminate it from the rest of that row or column outside the box.
- Type 2 (Box-Line Reduction): When a candidate in a row or column is confined to one box, eliminate it from the rest of that box.
- Always complete full pencil-marking before attempting to find Locked Candidates.
- This technique frequently triggers new naked singles or hidden singles, creating a productive chain of deductions.
- Systematic scanning — box by box and line by line — is the most reliable way to spot these patterns consistently.
- Mastering Locked Candidates dramatically extends the range of hard puzzles you can solve through pure logic.
Every great Sudoku solver started exactly where you are right now — learning one technique at a time, building a toolkit that makes hard puzzles feel manageable. Locked Candidates is one of those techniques that genuinely changes the game. The next time you pick up a challenging puzzle, take a few extra minutes to pencil in your candidates and scan for these hidden interactions between boxes and lines. You might be surprised at how quickly a grid that looked impossible starts to open up. Keep practicing, stay curious, and enjoy the journey — every puzzle solved is a small victory worth celebrating.