Locked Candidates in Sudoku: Pointing & Claiming

Locked Candidates let you remove a digit by combining two units at their intersection.

You do not know the exact cell that will contain the digit. What you know is more subtle:

Every possible position for this digit in one unit lies inside the same overlap with another unit.

That restriction is enough to eliminate the digit elsewhere.

There are two directions:

Pointing: box → row or column.
Claiming / Box-Line Reduction: row or column → box.

Sudoku visualPointing and claiming are mirror deductionsThe same intersection rule works in two directions: a box can lock a digit onto a line, or a line can lock it into a box.

Pointingbox → row

Claimingrow → box

Source candidates
Eliminations

The rule in one sentence

Pointing

If every candidate for digit X inside a box lies on the same row or column, remove X from that line outside the box.

Claiming

If every candidate for digit X inside a row or column lies in the same box, remove X from that box outside the line.

The logic is identical. Only the starting unit changes.

Why Locked Candidates work

Suppose box 5 still needs digit 6.

Inside that box, 6 can appear only in two cells, and both are on row 4.

You cannot yet say which source cell is 6.

But one of them must be 6 because the box still needs that digit.

Therefore row 4 will receive its 6 somewhere inside box 5.

Any other candidate 6 on row 4 outside box 5 would then duplicate the digit in the row.

So those outside candidates are impossible.

This is the key intermediate idea:

A digit can be forced into a region of the grid before it is forced into one exact cell.

That regional restriction is enough to make progress.

Pointing: box → line

Start with a box.

Ask:

Where can digit X still go in this box?

If every remaining position shares one row, the box points along that row.

If every remaining position shares one column, the box points down that column.

Claiming: line → box

Now reverse the question.

Start with a row or column.

Ask:

Where can digit X still go in this line?

If all remaining positions lie inside one box, the line claims the digit for that box.

The source unit must be complete

Locked Candidates are only valid when you have identified all remaining positions for the digit in the source unit.

For Pointing:

Are these genuinely all possible Xs in the box?

For Claiming:

Are these genuinely all possible Xs in the row or column?

If a legitimate candidate was accidentally omitted from your notes, you can create a false pattern.

That is why accurate candidates become increasingly important at Intermediate level.

Pointing does not require exactly two candidates

The name Pointing Pair is common, but the general logic can also use three cells.

Inside one box, the intersection with a particular row contains at most three cells.

If digit 2 can appear in all three of those cells — and nowhere else in the box — the box still guarantees that row will receive digit 2 inside the box.

The elimination outside the box remains valid.

VeyraPlay therefore uses:

Pointing Pairs as the familiar public page title;
Pointing Candidates when explaining the general principle.

What Locked Candidates usually unlock

The direct result is normally an elimination, not a placement.

But look at what can follow:

Locked Candidates
↓
remove candidate 6
↓
cell becomes {2}
↓
Naked Single 2
↓
peer candidates update
↓
new Hidden Single

This is why Intermediate Sudoku feels different from Beginner Sudoku: the useful move may be one or two logical steps away from the next visible digit.

How to search efficiently

Locked Candidates are easier to find digit-first.

Pointing scan

Choose one digit.
Move through each box that still needs it.
Look at the remaining candidate positions.
Ask whether they all share one line.
If yes, inspect that line outside the box.

Claiming scan

Keep the same digit selected.
Move through rows and columns with few remaining positions.
Ask whether all positions fall inside one box.
If yes, inspect the rest of that box.

This is more reliable than staring at every candidate set at once.

Locked Candidates vs Hidden Single

These techniques differ by how tightly the digit is restricted.

Hidden Single

One digit has one possible cell in the unit.

→ Place the digit.

Locked Candidates

One digit has multiple possible cells, but those cells are confined to one box-line intersection.

→ Eliminate the digit elsewhere.

A useful mental progression is:

one position
→ Hidden Single

multiple positions on one intersection
→ Locked Candidates

Locked Candidates vs subsets

Locked Candidates follow one digit across overlapping units.

Pairs and Triples follow several digits reserved for several cells inside one unit.

If a candidate filter on one digit looks interesting, intersections are a good search family.

If several small candidate sets look related, subsets may be the better family.

Common mistakes

Eliminating from the source cells

Those are the cells that must keep the digit.

Pointing eliminates from the line outside the box.

Claiming eliminates from the box outside the line.

Assuming “pair” means exactly two cells

Pointing can also use three aligned source cells.

Using only part of the source unit

The restriction must describe all valid positions for the digit in that source unit.

Treating the pattern as a solved placement

Locked Candidates tell you where the digit cannot go elsewhere. They do not usually decide which source cell is true.

Applying a pattern that produces no elimination

The restriction can exist without any candidate to remove. It is logically true, but it is not currently a productive solving step.

A useful verbal check

Before deleting anything, say the deduction in one sentence:

“The 5 for this box must be somewhere on row 6, so row 6 cannot contain 5 anywhere else.”

or:

“The 8 for this column must be inside this box, so the rest of the box cannot contain 8.”

If that sentence is not fully justified by the candidate state, do not make the elimination.

FAQ

Are Pointing Pairs the same as Locked Candidates?

Pointing is one of the two main Locked Candidates directions. Claiming / Box-Line Reduction is the other.

What are Locked Candidates Type 1 and Type 2?

A common solver naming convention is:

Type 1 = Pointing;
Type 2 = Claiming.

Can Locked Candidates use a column?

Yes. Every row-based explanation has a column equivalent.

Can Locked Candidates solve a cell immediately?

Usually the direct move is an elimination, but the elimination may create a Single.

Should I learn Pointing and Claiming separately?

Yes. The umbrella logic is the same, but training the two search directions separately makes the pattern much easier to recognize.

What to learn next

Study Pointing Pairs and Box-Line Reduction as separate search directions, then move into Naked Pairs and Hidden Pairs.

techniques · intermediatePointing PairsLearn Pointing Pairs and Pointing Candidates: when a digit is locked to one line inside a box and can be removed elsewhere on that line.techniques · intermediateBox-Line ReductionLearn Box-Line Reduction, also called Claiming, and use a row or column that is confined to one box to eliminate candidates inside it.techniques · intermediateNaked PairLearn how Naked Pairs reserve two digits for two cells and eliminate those candidates from other cells in the same row, column or box.techniques · intermediateHidden PairLearn how Hidden Pairs work when two digits are restricted to the same two cells, and remove the extra candidates hiding the pair.strategies · intermediateIntermediate Sudoku StrategiesLearn how to progress beyond Singles using candidates, Locked Candidates, Pairs, Triples and a structured intermediate solving workflow.