Locked Candidates in Sudoku: How Box-Line Reduction Really Works

Locked candidates in Sudoku are one of the first intermediate techniques that make a puzzle feel easier without requiring advanced pattern hunting. If you want the short version, locked candidates happen when all possible spots for one digit are restricted to the overlap between a 3×3 box and a single row or column. That restriction lets you remove the same digit from other cells outside the overlap.

Many players already use this idea without knowing the name. You may have heard it called box-line reduction, intersection removal, pointing pairs, or claiming. Those labels describe the same family of logic from slightly different angles.

This guide explains what locked candidates are, how to spot them, when to use them, and how they connect to the rest of a strong Sudoku solving routine.

Quick Answer: What Are Locked Candidates in Sudoku?

Locked candidates in Sudoku appear when all possible positions for a digit in one box fall on the same row or column, or when all possible positions for a digit in one row or column fall inside the same box. In either case, the digit is “locked” into that overlap, so you can eliminate it from the rest of the connected unit.

Featured snippet answer: Locked candidates are a Sudoku elimination technique where a candidate is confined to the intersection of a box and a row or column. This allows you to remove that candidate from other cells in the row, column, or box.

Why Locked Candidates Matter

Locked candidates matter because they often appear before harder strategies like X-Wing, chains, or coloring. They do not always solve a cell immediately, but they simplify the grid, clean up notes, and create new singles and pairs.

In practice, this technique is one of the best bridges between beginner Sudoku and intermediate Sudoku. If you already know naked singles, hidden singles, and basic note-taking, locked candidates are usually the next technique worth learning.

The Two Types of Locked Candidates

There are two common forms of locked candidates in Sudoku. Different guides use different names, but the underlying logic is the same.

1. Pointing pairs and pointing triples

Inside one 3×3 box, all candidates for a digit may lie on the same row or the same column. When that happens, the digit cannot appear anywhere else in that row or column outside the box.

Example: in the top-left box, the digit 6 appears only in two cells, and both cells are in row 2. That means row 2 cannot contain another 6 outside that box.

This is often called a pointing pair when there are two cells, or a pointing triple when there are three.

2. Claiming pairs and claiming triples

The reverse pattern also matters. In one row or column, all candidates for a digit may fall inside the same 3×3 box. When that happens, the digit cannot appear elsewhere inside that box outside the row or column.

Example: in row 5, every possible 8 lies inside the center box. That means the other unsolved cells in the center box cannot contain 8.

This is commonly called claiming or box-line reduction.

How to Spot Locked Candidates Step by Step

Start with one digit at a time

Pick a candidate such as 4 or 7 and scan box by box. Locked candidates are easier to see when you focus on a single digit instead of trying to read every candidate in the grid at once.

Check whether the candidate is confined to one line

Inside a box, ask: are all possible placements for this digit on the same row or column? If yes, you probably have a pointing pattern.

Inside a row or column, ask: do all possible placements for this digit fall inside one box? If yes, you probably have a claiming pattern.

Make only the elimination the pattern proves

Locked candidates do not always solve a number right away. Sometimes the only correct move is removing one or two candidates elsewhere. That is still progress. Many Sudoku mistakes come from expecting every technique to produce an immediate placement.

A Simple Locked Candidates Example

Imagine the top-middle 3×3 box. The digit 9 can only go in two cells, and both are on column 5.

That means the box locks 9 into column 5.
Because of that, every other unsolved cell in column 5 outside the box can remove 9 as a candidate.

You may not place a final number immediately, but that elimination can turn another cell into a hidden single or clean up a pair elsewhere in the grid.

Pointing vs Claiming: What Is the Difference?

The easiest way to remember the difference is the direction of the logic.

Pointing: start inside a box and eliminate from a row or column.
Claiming: start inside a row or column and eliminate from a box.

They are both locked candidates in Sudoku. The only difference is which unit you notice first.

When Should You Look for Locked Candidates?

Use locked candidates after basic singles are exhausted and after your notes are reasonably complete. For many medium puzzles, the best solving loop looks like this:

Scan for naked singles.
Scan for hidden singles.
Check locked candidates.
Check pairs.
Only then move to harder patterns if the grid still resists.

This order matters because locked candidates are cheaper to verify than many advanced techniques. They often unlock the puzzle before you need anything more complex.

Common Mistakes With Locked Candidates

Mixing up the units. Always confirm the digit is truly confined to one row or one column within the box, or one box within the row or column.
Eliminating too much. Remove the candidate only from the unit the pattern affects. Do not erase candidates from unrelated cells.
Using stale notes. Locked candidates are hard to spot when pencil marks are outdated.
Skipping easier logic. If obvious singles still exist, take them first. Locked candidates are powerful, but they are not the first thing to check every turn.

Are Locked Candidates a Beginner Technique?

They sit right at the edge of beginner and intermediate Sudoku. A brand-new player can understand the logic, but the pattern becomes much easier once you are comfortable with candidates and notes.

If you are still learning how to maintain pencil marks, start there first. Once notes make sense, locked candidates usually click quickly because the logic is clean and visual.

What Comes After Locked Candidates?

Once you can spot locked candidates consistently, the next useful techniques are usually:

naked pairs,
hidden pairs, and
simple fish patterns such as X-Wing.

But do not rush ahead. Many medium Sudoku puzzles open up completely once you apply singles, locked candidates, and one or two pair eliminations with discipline.

FAQ: Locked Candidates in Sudoku

Are locked candidates and box-line reduction the same thing?

Nearly always, yes. Many guides use box-line reduction as another name for locked candidates, especially when describing claiming-type eliminations.

Are pointing pairs and claiming both locked candidates?

Yes. They are the two main forms of locked candidates in Sudoku. Pointing starts from the box. Claiming starts from the row or column.

Do locked candidates solve cells directly?

Sometimes, but not always. Their main job is to eliminate candidates. Those eliminations often create a hidden single, naked single, or pair later.

When should I check for locked candidates?

Check for them after singles and before more advanced pattern hunting. They are one of the most efficient intermediate techniques in standard Sudoku.

Conclusion: Learn Locked Candidates Early

Locked candidates in Sudoku are worth learning early because they deliver a lot of value for relatively little complexity. Once you understand how box-and-line overlap works, pointing and claiming stop feeling like separate tricks and start feeling like one clear logical family.

If you want to improve without guessing, practice this order: singles first, then locked candidates, then pairs. That routine solves far more puzzles than most players expect. For more step-by-step strategy help, keep practicing on Pure Sudoku and compare this technique with pointing pairs, claiming, and how to use notes in Sudoku.