Candidate Lines Sudoku: What They Are and How They Remove Candidates
Candidate lines Sudoku is a beginner-friendly way to describe a locked-candidates pattern. It happens when all possible positions for one digit inside a 3×3 box fall on the same row or the same column. When that happens, you can remove that digit from the rest of that row or column outside the box.
If that sentence sounds abstract, do not worry. This guide breaks down candidate lines Sudoku in plain English, shows why the pattern works, and explains how it connects to terms like locked candidates, box-line reduction, and pointing pairs.
Quick Answer: What Are Candidate Lines in Sudoku?
Candidate lines in Sudoku appear when one candidate in a box is restricted to a single row or column. Because that digit must stay inside the box, it cannot also appear elsewhere in that same row or column outside the box. You can eliminate those outside candidates safely.
Why Candidate Lines Matter
This pattern matters because it turns scattered pencil marks into a clear elimination. Instead of checking every cell one by one, you use the relationship between a box and a line to remove impossible candidates fast.
Candidate lines are useful because they:
- unlock medium puzzles when singles stop appearing,
- clean up pencil marks before harder patterns, and
- teach you to see rows, columns, and boxes as connected logic rather than separate zones.
How Candidate Lines Sudoku Works
Look at one digit, such as 6, inside a single 3×3 box.
If every possible 6 in that box sits on the same row, one of those cells must eventually contain the 6. That means no other cell in that row outside the box can be 6.
The same idea works vertically. If all candidate 6s in a box sit on the same column, remove 6 from the rest of that column outside the box.
Simple row example
Imagine the top-left box has only two possible places for digit 4, and both of them are in row 2. Since the 4 for that box has to be in row 2, every other 4 candidate in row 2 outside that box can be erased.
Simple column example
Now imagine a middle box where every possible 8 lies in column 5. The 8 for that box must land somewhere in column 5, so any other 8 candidate in column 5 outside the box can be removed.
Candidate Lines vs Locked Candidates
In most Sudoku guides, candidate lines is not a separate technique. It is another way of talking about part of locked candidates in Sudoku.
Here is the cleanest way to think about the terms:
- Locked candidates is the broader family name.
- Candidate lines describes the visual cue: the candidates line up in one row or column inside a box.
- Pointing pairs and pointing triples are common subtypes when two or three candidates create the line.
- Box-line reduction is a closely related label some solvers use for the same interaction.
If different sites use different names, the underlying logic is usually the same: a candidate is restricted in one unit, so it can be removed from another connected unit.
How to Spot Candidate Lines Faster
1. Scan one digit at a time
Do not stare at every pencil mark at once. Pick one digit, such as 3, and check where it can still go inside each box.
2. Look for all candidates in one box landing on one row or column
This is the key trigger. You are not looking for solved numbers. You are looking for candidate positions that line up.
3. Check the matching row or column outside the box
Once the alignment appears, scan the rest of that row or column. Any matching candidate outside the box can be removed.
4. Re-scan for singles immediately
Candidate lines often create a new candidate shortage somewhere else. After each elimination, check for naked singles, hidden singles, or a cleaner notes layout.
Common Candidate Lines Mistakes
- Eliminating inside the box instead of outside it: the restriction tells you what to remove from the connected row or column beyond the box.
- Using solved digits instead of candidates: this is a pencil-mark technique, not just a visual placement trick.
- Forgetting the line must be complete inside one box: if the candidate appears in more than one row or column inside that box, the pattern is not valid.
- Mixing the direction: if the candidates line up on a row, eliminate on that row; if they line up on a column, eliminate on that column.
Candidate Lines vs Pointing Pairs
Pointing pairs are one specific version of candidate lines. If a box has exactly two candidate cells for one digit and they share the same row or column, that is a pointing pair. If there are three aligned candidates, many solvers call it a pointing triple.
So if you already know pointing pairs in Sudoku, you already understand one of the most common candidate-lines patterns.
Candidate Lines vs Box-Line Reduction
The phrase box-line reduction is often used interchangeably with candidate lines, although some solvers prefer it for the opposite direction of the same box-line interaction. In practical solving, both ideas belong to the same logic family.
If you want the broader version next, study box-line reduction Sudoku after this article.
When to Use Candidate Lines in a Real Puzzle
Use this technique when:
- basic singles have dried up,
- you already have clean pencil marks, and
- one box looks crowded but one digit keeps lining up on the same row or column.
This often happens in easy-to-medium grids and shows up regularly in harder puzzles as a setup move before more advanced eliminations.
FAQ: Candidate Lines Sudoku
What are candidate lines in Sudoku?
Candidate lines in Sudoku happen when all possible cells for one digit inside a 3×3 box lie on the same row or column. That lets you eliminate the digit from the rest of that row or column outside the box.
Is candidate lines the same as locked candidates?
Usually yes in practical terms. Candidate lines is a plain-English way to describe a locked-candidates pattern, especially the pointing form.
Are candidate lines a beginner or advanced technique?
They are usually taught as an early intermediate technique. The logic is simple once you are comfortable using notes.
Do candidate lines work without pencil marks?
Not reliably. You usually need candidate notes to see the alignment clearly and make safe eliminations.
What is the difference between candidate lines and pointing pairs?
Pointing pairs are a specific kind of candidate-lines pattern where exactly two candidates in a box align on the same row or column.
Conclusion
Candidate lines Sudoku is one of the clearest ways to understand locked candidates. When a digit is confined to one row or column inside a box, the rest of that line loses the right to hold that digit.
If you want to build this into your solving routine, practice it alongside locked candidates, pointing pairs, and stronger note-taking. The more cleanly you track candidates, the easier this pattern becomes to spot.