Locked Candidates

Locked Candidates are intersection techniques that use the overlap between a 3×3 box and a row or column.

You do not usually know the exact cell that contains the digit. Instead, you prove that every possible position for one digit in a source house is trapped inside the same box-line intersection.

That partial restriction is enough to eliminate the digit from the other side of the intersection.

There are two mirror directions:

• Pointing / Locked Candidates Type 1: box → row or column.
• Claiming / Locked Candidates Type 2: row or column → box.

Locked Candidates at a glance

The two rules are easiest to remember by asking where the restriction starts.

Pointing: box → line

If every candidate for digit X inside one box lies on the same row or column, X can be removed from that line outside the box.

Claiming: line → box

If every candidate for digit X inside one row or column lies in the same box, X can be removed from that box outside the line.

The proof is the same in both directions. Only the source house changes.

Why Locked Candidates work

Suppose box 5 still needs digit 6.

Inside that box, every legal position for 6 lies on row 4.

You may not know which of those cells is 6, but one of them must eventually contain 6 because the box requires one 6.

Therefore row 4 is guaranteed to receive its 6 inside box 5.

A second 6 anywhere else on row 4 would violate the row constraint, so candidate 6 can be removed from row 4 outside the box.

Claiming reverses the same argument. If every possible 6 in row 4 lies inside box 5, then row 4 guarantees that box 5 receives its 6 from the intersection. Candidate 6 can therefore be removed from the rest of box 5.

The important idea is:

Sudoku can force a digit into a region before it forces the digit into one exact cell.

That regional certainty is already usable information.

The geometry: a box-line intersection

A Locked Candidates pattern always depends on two houses that overlap:

• one box;
• one row or one column crossing that box.

Their overlap contains at most three cells.

The source candidates must all lie inside that overlap. The elimination targets lie in the part of the target house outside the overlap.

This is why Locked Candidates are sometimes described more generally as intersection removal.

Pointing: the box restricts the line

Start with a box and one digit.

Ask:

Where can digit X still go in this box?

If every remaining X lies on one row, the box points along that row. If every remaining X lies in one column, the box points down that column.

The digit can then be removed from the shared line outside the box.

Claiming: the line restricts the box

Now reverse the search.

Start with a row or column and one digit.

Ask:

Where can digit X still go in this line?

If every remaining X lies inside one box, the line claims that box's copy of X.

The digit can then be removed from the rest of the box outside the line.

Pointing and Claiming can use two or three source cells

Names such as Pointing Pair can make the pattern sound more restrictive than it is.

A box-line intersection contains up to three cells. If all remaining positions for X in the source house are confined to that intersection, the logic works whether there are:

• two source positions;
• three source positions.

What matters is not the number of source cells. What matters is that every legal source position lies in the same overlap.

That is why Pointing Candidates is a useful general name for the Pointing direction.

The source house must be complete

This is one of the most important verification rules in Intermediate Sudoku.

For Pointing, ask:

Have I accounted for every legal X in this box?

For Claiming, ask:

Have I accounted for every legal X in this row or column?

If one legitimate source candidate exists outside the intersection, the lock is broken and the elimination is invalid.

This is especially important when using displayed candidates. Incomplete or stale notes can manufacture a false Locked Candidates pattern.

Locked Candidates are usually eliminations, not placements

The direct move is normally:

remove candidate X.

That may feel less dramatic than filling a cell, but eliminations can immediately create placements.

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

This is one of the first places where Sudoku solving clearly shifts from placing forced digits to creating future placements through candidate elimination.

How to find Locked Candidates efficiently

Locked Candidates are usually easier to search digit-first.

Search for Pointing

1. Choose one digit.
2. Inspect each box that still needs that digit.
3. Identify all remaining positions inside the box.
4. Check whether they all share one row or column.
5. If they do, inspect that line outside the box for eliminations.

Search for Claiming

1. Keep the same digit selected.
2. Inspect rows and columns with few remaining positions.
3. Check whether all positions fall inside one box.
4. If they do, inspect the rest of that box for eliminations.

Digit filters in digital Sudoku make this search much easier because the pattern is about the distribution of one candidate value, not the full notes inside every cell.

Locked Candidates vs Crosshatching

Crosshatching and Locked Candidates are closely connected.

Crosshatching asks where one digit can still go, often inside a box.

If the restrictions leave exactly one cell:

→ you may have a Hidden Single.

If several cells remain but all lie on one line:

→ you may have Pointing.

So Locked Candidates are a natural extension of the same positional thinking used in beginner scanning.

Locked Candidates vs Hidden Single

The difference is how tightly the digit is restricted.

Hidden Single

One digit has one possible cell in a house.

→ Place the digit.

Locked Candidates

One digit has multiple possible cells, but all of them are trapped in one box-line intersection.

→ Eliminate the digit from the other side of the intersection.

A useful progression is:

one possible position
→ Hidden Single

multiple positions in one intersection
→ Locked Candidates

Locked Candidates vs subsets

Locked Candidates and subsets are both reservation ideas, but they reserve different structures.

Locked Candidates

Follow one digit across overlapping houses.

X must be somewhere in this intersection.

Naked or Hidden subsets

Follow several digits and several cells inside one house.

These digits are reserved for this set of cells.

If one candidate value forms an interesting geometric restriction, check intersections. If several candidate sets seem to reserve each other, check subsets.

Productive pattern vs true-but-useless pattern

A lock can be logically true even when it removes nothing.

For example, every candidate 7 in a box may lie on row 5, but if row 5 has no other candidate 7 outside the box, there is no immediate elimination.

The restriction is still correct. It is simply not a productive solving step at that moment.

When searching manually, prioritize patterns that actually change the candidate state.

Common mistakes

Eliminating from the source cells

The source candidates are the positions that make the lock true. They must remain.

Pointing eliminates from the line outside the box. Claiming eliminates from the box outside the line.

Treating “pair” as a requirement

Two or three source positions can support the same intersection logic.

Ignoring an extra source candidate

All legal source positions must be confined to the same intersection.

Treating the lock as a solved digit

You know the region that contains X, not usually the exact source cell.

Using incomplete candidates as proof

A missing legitimate candidate can create a false pattern.

Making an elimination that changes nothing

A correct restriction with no target candidate to remove is not useful yet.

A verbal proof check

Before deleting a candidate, say the deduction in one sentence.

For Pointing:

“This box must place 5 somewhere on row 6, so row 6 cannot contain 5 outside this box.”

For Claiming:

“This column must place 8 inside this box, so the rest of the box cannot contain 8.”

If you cannot justify every part of the sentence from the current grid, 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 convention, including HoDoKu, uses Type 1 for Pointing and Type 2 for Claiming. Other resources may prefer the descriptive names instead.

Can Locked Candidates work with a column?

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

Do Locked Candidates require exactly two candidates?

No. The source positions can occupy two or three cells in the relevant box-line intersection.

Can Locked Candidates solve a cell immediately?

The direct move is normally an elimination, but that elimination can create a Naked Single, Hidden Single, or another useful pattern.

Should I learn Pointing and Claiming separately?

Yes. The proof is mirrored, but training the two search directions separately makes recognition much faster.

What to learn next

Study Pointing Pairs / Pointing Candidates and Box-Line Reduction / Claiming separately until both directions are automatic.

Then move into Naked Pairs and Hidden Pairs, where the reservation changes from one digit in an intersection to several digits reserved for several cells.