The idea
These two moves are usually taught separately but they are the same observation from either side. Both are called *locked candidates*, because a digit is locked into the overlap between a box and a line.
Pointing pairs — from box to line
If a digit can only go in cells of one box that all sit in the same row, then that digit must be somewhere in that row within the box. So it cannot be anywhere else in that row, outside the box. Strike it from those cells.
The digit is confined to a single line inside its box, so the marked cells further along that line can no longer take it.
Box/line reduction — from line to box
The mirror image. If a digit can only go in cells of one row that all sit in the same box, then that digit must be in the box inside that row — so it can be struck from the rest of the box.
Why it matters
Locked candidates rarely place a digit by themselves. What they do is remove candidates, and those removals usually create a naked single or a hidden single somewhere else. This is the first technique where you eliminate before you place, and it is the dividing line between medium and easier puzzles.
How to spot them
- Look at a box and pick a digit that is missing from it. If its possible cells all share a row or a column, you have a pointing pair.
- Two candidate cells are enough; three in a line work the same way.
- Check every band of three boxes after a placement — this is where locked candidates appear most often.
Practise on medium puzzles, which are built to need exactly this move.
Common questions
What is a pointing pair in Sudoku?
Two or three cells in one box that are the only places a digit can go, and that all lie in the same row or column. The digit can then be removed from the rest of that row or column.
What is box/line reduction?
The reverse case: a digit in a row or column whose only possible cells all lie in one box, letting you remove it from the rest of the box.
Are these the same as locked candidates?
Yes. Locked candidates is the umbrella term for both directions.
Does a pointing pair place a digit?
Not directly. It removes candidates, and the removal usually creates a single that does place one.