Candidate Lines Sudoku: How Box-to-Line Logic Eliminates Candidates

If your pencil marks are getting crowded and singles have dried up, candidate lines Sudoku is one of the most useful next techniques to learn. It is not flashy, but it clears clutter fast. When a candidate is confined to one line inside a 3×3 box, that same candidate can be removed from the rest of the line outside the box.

Quick answer: Candidate lines in Sudoku happens when all possible places for one digit inside a box lie in a single row or a single column. That means the digit must stay inside that box on that line, so you can eliminate the same digit from the rest of the row or column outside the box.

This guide explains how candidate lines works, why it is part of locked candidates logic, and how to spot it without staring at every note in the grid.

What is candidate lines in Sudoku?

Candidate lines is a box-and-line interaction. You look inside one 3×3 box and track a single digit. If every possible spot for that digit in the box sits on the same row or the same column, the digit is locked there.

Because the digit must appear in that box somewhere on that one line, it cannot also appear elsewhere on the same row or column outside the box.

You may also see this idea called:

• box line reduction,
• locked candidates type 1, or
• pointing logic, depending on the terminology a solver uses.

The label matters less than the logic. The core idea is that the box forces a restriction onto the row or column.

Why candidate lines Sudoku works

Suppose digit 6 can appear in only two cells inside the top-left box, and both of those cells sit on row 2. Then row 2 must contain the 6 in that box. There is no reason to know which of the two cells is correct yet. But you do know something important: row 2 cannot contain another 6 outside that box.

That is why candidate lines Sudoku produces eliminations instead of direct placements. It narrows the grid by proving where a digit cannot go.

Candidate lines Sudoku example in plain English

Imagine the middle-left 3×3 box. You check candidate 4 and see that the only two places for 4 in that box are:

• row 5, column 1
• row 5, column 3

Both candidates are on row 5. That means row 5 must place its 4 somewhere inside this box. So any other 4s on row 5 outside this box can be removed.

You are not solving the exact cell yet. You are cleaning row 5 because the box has already claimed the digit on that line.

How to find candidate lines Sudoku step by step

1. Pick one box, not the whole grid

Candidate lines is easiest to see when you stay local. Scan one 3×3 box at a time.

2. Track one digit inside that box

Do not compare all notes at once. Choose a single digit and check every unsolved cell in the box.

3. Ask whether all remaining candidates share one row or one column

If the digit appears only on one row inside the box, or only on one column inside the box, you have candidate lines.

4. Eliminate the same digit outside the box on that line

Remove the digit from the rest of the affected row or column, but only outside the box that created the lock.

5. Re-scan for singles and pairs immediately

Candidate lines often exposes a hidden single, naked single, or a cleaner pair right after the elimination. Do not move on too quickly.

Row-based vs column-based candidate lines

The logic is identical in both cases:

• Row-based candidate lines: the digit is locked to one row inside a box, so remove it from the rest of that row outside the box.
• Column-based candidate lines: the digit is locked to one column inside a box, so remove it from the rest of that column outside the box.

If you can learn to see one version clearly, the other version becomes automatic.

Candidate lines vs claiming vs pointing pairs

These terms confuse a lot of players because they all involve the same box-line relationship.

• Candidate lines: broad label for box-line restriction logic.
• Pointing pairs or pointing triples: the digit is restricted inside a box to one row or column, so you eliminate outside the box on that line.
• Claiming: the digit is restricted inside a row or column to one box, so you eliminate inside the box outside that line.

The direction changes, but the reasoning is the same: a box and a line are sharing control over where one digit can go.

If you want those neighboring techniques separately, see Claiming Sudoku, Pointing Pairs Sudoku, and Intersection Removal Sudoku.

When should you look for candidate lines?

Candidate lines is most useful when:

• obvious singles are slowing down,
• your notes are accurate enough to trust,
• one box has only two or three positions for a digit,
• you want a medium-strength technique before moving into chains or fish.

It is a very good bridge between beginner solving and true intermediate solving because the logic is clean, local, and repeatable.

A fast scanning routine for candidate lines Sudoku

1. Choose one box.
2. Mentally scan digits 1 through 9, one at a time.
3. Notice any digit whose candidates stay on one row or one column inside the box.
4. Eliminate that digit from the rest of the affected line.
5. Check whether the cleanup creates a single or pair immediately.

This is faster than reading every candidate list cell by cell. You are scanning for structure, not for raw clutter.

Common candidate lines Sudoku mistakes

Eliminating inside the box instead of outside it

The whole point is that the box locks the digit onto a line. Your eliminations happen on the rest of that row or column outside the box.

Using incomplete notes

If your candidates are sloppy, candidate lines becomes unreliable. This technique depends on correct pencil marks.

Forcing the pattern when candidates span two rows and two columns

If the digit is spread across multiple rows and columns inside the box, there is no candidate lines move. Do not invent a lock that is not there.

Ignoring follow-up moves

The elimination itself may look small, but the real value often appears one step later. Always re-check the affected row, column, and box after the cleanup.

Why candidate lines matters in your solving order

Many puzzles do not require exotic chains. They require good cleanup. Candidate lines matters because it teaches you to connect boxes with rows and columns instead of treating each unit in isolation.

For many solvers, the progression looks like this:

1. scanning and singles
2. pencil marks
3. naked and hidden pairs
4. candidate lines and claiming
5. stronger patterns like wings, chains, and fish

If you want a broader roadmap, read Intermediate Sudoku Techniques and Locked Candidates Type 1 vs Type 2.

FAQ: candidate lines Sudoku

What is candidate lines in Sudoku?

Candidate lines is a Sudoku technique where all candidates for one digit inside a box lie on the same row or column. That lets you remove the same digit from the rest of that line outside the box.

Is candidate lines the same as pointing pairs?

Pointing pairs and pointing triples are common forms of candidate lines. Some guides use the broader term candidate lines, while others name the pattern by how many candidates are involved.

Is candidate lines the same as claiming?

No. The logic is related, but the direction is reversed. Candidate lines starts from the box and affects the line. Claiming starts from the line and affects the box.

Do candidate lines solve a cell immediately?

Sometimes, but not usually. Most of the time the technique removes candidates and sets up the next move.

When should beginners learn candidate lines Sudoku?

Right after they are comfortable with notes, singles, and basic pairs. It is one of the cleanest intermediate techniques to learn early.

Conclusion

Candidate lines Sudoku is one of the best examples of practical logic: small idea, reliable payoff. When a box locks a digit onto one row or column, the rest of that line gets simpler immediately.

If you already use notes and want a smarter way to break through medium puzzles, practice candidate lines on a fresh grid at Pure Sudoku. Start with one box, track one digit, and let the structure do the work.