Locked Candidates Type 1 vs Type 2 in Sudoku

Locked candidates type 1 vs type 2 in Sudoku is really a comparison between the two classic box-line interactions. Type 1 is usually called pointing: candidates are locked inside one box and one line, so you can remove that digit from the rest of the line. Type 2 is usually called claiming: candidates are locked inside one line and one box, so you can remove that digit from the rest of the box.

If those definitions sound almost identical, that is the point. The logic is the same in both cases. What changes is the direction of the elimination.

This guide explains locked candidates type 1 vs type 2 in plain English, shows how each pattern works, and helps you decide which one to scan for first in a real puzzle.

What are locked candidates in Sudoku?

Locked candidates happen when all possible places for one digit are restricted to the overlap between a 3×3 box and a single row or column. Once that happens, the digit is effectively trapped in that overlap. Because it must go there somewhere, you can eliminate the same digit from other cells that share the affected row, column, or box.

This family of logic is sometimes called box-line reduction or intersection removal. Different Sudoku sources use different names, but the practical idea is the same: a digit cannot appear twice in the same unit, so a restriction in one place creates an elimination somewhere else.

Locked Candidates Type 1 vs Type 2 in Sudoku

Type 1: Pointing

In locked candidates type 1 Sudoku, all candidates for one digit inside a single 3×3 box fall on the same row or the same column. Because that digit must be placed in that box on that line, the rest of the line outside the box cannot contain that digit.

Shortcut definition: box to line.

Example:

Inside the top-left box, the digit 7 appears only in three cells of row 2.
That means one of those three cells must be the 7 for the box.
So every other 7 candidate in row 2 outside that box can be removed.

This is why many solvers call type 1 pointing pairs or pointing triples, depending on whether there are two or three candidates lined up inside the box.

Type 2: Claiming

In locked candidates type 2 Sudoku, all candidates for one digit in a row or column are contained inside the same 3×3 box. Because that digit must appear on that line within that box, the rest of the box cannot contain that digit.

Shortcut definition: line to box.

Example:

In row 5, every candidate 4 is located inside the middle box.
That means row 5 will place its 4 somewhere in that box.
So any other 4 candidates in the same box but not on row 5 can be removed.

This is usually called claiming because the row or column is effectively claiming that digit inside one specific box.

The simplest way to remember the difference

If you keep mixing up locked candidates type 1 vs type 2 in Sudoku, use this test:

If you start by looking inside a box and then eliminate from a line, it is type 1.
If you start by looking along a line and then eliminate from a box, it is type 2.

That one distinction clears up most of the terminology confusion.

Why both patterns matter

Neither pattern is flashy, but both are efficient. Locked candidates often appear early in medium and hard puzzles, and they regularly open the board without forcing you into advanced chain logic.

They are especially useful when:

you have already filled easy singles but the grid still feels crowded
several boxes have three or four candidate copies of the same digit
you want progress without guessing

Many solvers improve simply by scanning for locked candidates more consistently. They are one of the best bridges from beginner Sudoku into intermediate solving.

A quick side-by-side example

Type 1 example

Suppose the digit 9 in the bottom-right box appears only at r7c8, r7c9, and r7c7. All three candidates lie on row 7. Since the box must place its 9 somewhere on row 7, no other cell in row 7 outside that box can still be a 9.

Type 2 example

Now imagine row 3 has candidate 2 only at r3c4 and r3c6, and both cells sit in the top-middle box. Since row 3 must place its 2 inside that box, every other candidate 2 in the top-middle box outside row 3 can be eliminated.

Notice what changed:

Type 1 started from the box and eliminated from the row.
Type 2 started from the row and eliminated from the box.

How to scan for locked candidates faster

1. Pick one digit at a time

Instead of looking for every possible pattern at once, scan the grid digit by digit. For example, check all candidate 6s across the puzzle. This makes box-line interactions easier to spot.

2. Look for crowded boxes first

Type 1 usually shows up when a box has two or three copies of the same candidate lined up on one row or column. If a box looks busy, inspect it before scanning the rest of the puzzle.

3. Use rows and columns as filters

Type 2 is easier to find when a row or column has only a few remaining positions for one digit. If all of those positions fall into one box, you have a claiming elimination.

4. Recheck the affected unit immediately

After each elimination, look again at the row, column, and box you just changed. Locked candidates often create a hidden single or another quick follow-up.

Common mistakes when using locked candidates

Eliminating from the wrong unit. Type 1 eliminates from the rest of a row or column. Type 2 eliminates from the rest of a box.
Forgetting that all candidates must be restricted. If even one extra candidate for the digit exists outside the pattern, the logic does not hold.
Mixing up confirmed digits and candidates. Locked candidates are pencil-mark logic. You are not proving the digit’s exact location, only where it cannot go.
Skipping the column version. Many players notice row-based patterns faster and miss the same logic in columns.

Where locked candidates fit in your solve order

A practical order for many solvers looks like this:

Fill obvious singles.
Scan for hidden singles.
Check locked candidates type 1 and type 2.
Look for pairs, triples, or more advanced eliminations if needed.

This is one reason locked candidates deserve focused practice. They are early enough to matter often, but strong enough to break open puzzles that no longer yield easy singles.

FAQ

Are locked candidates type 1 and pointing pairs the same thing?

Pointing pairs are a common form of locked candidates type 1. If there are three aligned candidates instead of two, many solvers call it a pointing triple, but the underlying logic is still type 1.

Are locked candidates type 2 and claiming the same thing?

Yes. In most Sudoku guides, locked candidates type 2 is the same as claiming.

Is one type more important than the other?

No. They are equally useful. Some puzzles reveal more type 1 opportunities, while others give clearer type 2 eliminations.

Do locked candidates solve the cell directly?

Usually not. They remove bad candidates and make later singles or subset patterns easier to find.

Conclusion

The cleanest way to understand locked candidates type 1 vs type 2 in Sudoku is to focus on direction. Type 1 goes from box to line. Type 2 goes from line to box. Once you see that, the names become much less confusing, and the eliminations become faster to trust in real puzzles.

If you want to build this technique into your regular solving routine, study it alongside pointing pairs in Sudoku, claiming Sudoku, and intersection removal Sudoku. Those pages complete the same logic family from slightly different angles.