Pick a box and a digit, and find every cell where the digit is still a candidate.
Pointing Pairs
Locked candidates — when a box points a digit down a single line.
A pointing pair (or triple) is a locked candidate. If, within a box, a digit's only possible cells all lie in the same row or column, then that digit must be in the box — and therefore can be removed from the rest of that row or column outside the box.
Worked example
5
5
1
8
9
4
How to apply it
Check whether all of those cells share a single row or column.
If they do, the digit is locked to that line within the box.
Remove the digit from that row or column in the other two boxes it passes through.
When to use it
Pointing pairs and the reverse pattern, box-line reduction, are essential on hard and expert puzzles. They rarely place a digit directly but trim candidates so that singles reappear.
Frequently asked questions
What is box-line reduction?
It is the mirror of a pointing pair: when a digit in a row or column is confined to one box, you remove it from the rest of that box.