Locked Candidates (general form)

Also called Intersection Removal, Line-Box Interaction, Locked Candidate.

New to this shorthand? How to read a sudoku grid explains r4c7, houses, candidates and the rest.

What it means

Sometimes all the spots for a digit inside one group also sit inside a second group. Then the digit must land where the two groups overlap. So you can rub it out everywhere else in that second group.

The exact rule, for stronger players

The rule, stated exactly: For two houses A and B whose intersection is non-empty, if every candidate of digit d in A lies within A intersect B, then d may be eliminated from B minus A.

What it removes: Remove d from B minus A.

Why it works: d must be placed in A, and every place it can go is also in B. Therefore d is placed in B at that intersection, and B admits d only once.

Worked examples

These come from real Proven Sudoku boards. Nobody drew them by hand to make the pattern look neat. Each one is checked first, so the board is one you could really meet, the pattern is really there, and every digit it rules out is really wrong.

Sudoku grid showing Locked Candidates (general form). Every 2 in b4 lies in r4, so the 2 for that box is somewhere on that line and can be removed from the rest of it. — Every 2 in b4 lies in r4, so the 2 for that box is somewhere on that line and can be removed from the rest of it.

Sudoku grid showing Locked Candidates (general form). Every 1 in b3 lies in c9, so the 1 for that box is somewhere on that line and can be removed from the rest of it. — Every 1 in b3 lies in c9, so the 1 for that box is somewhere on that line and can be removed from the rest of it.

Learn these first

Hidden SingleLook at one digit in one row, column or box.

Related locked candidates

Locked Candidates Type 1 (Pointing)Inside one box, every spot left for a digit sits in the same row or column.
Locked Candidates Type 2 (Claiming)Inside one row or column, every spot left for a digit sits in the same box.

How often does it come up?

Our rough estimate: umbrella page. Pointing and claiming are the two orientations of one idea.

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