Hidden Pairs

What is a hidden pair?

A hidden pair happens when two digits can each go in only two cells of a row, column, or box, and it is the same two cells for both digits. Those two cells must hold those two digits, so any other candidate written in them can be erased.

How to find a hidden pair

1. Take one unit with complete candidate notes.
2. For each missing digit, count its possible cells. You are looking for digits that appear exactly twice.
3. Match them up. If two such digits appear in the very same two cells, you have a hidden pair.
4. Clean the cells. Remove every other candidate from those two cells.

In the example, column 1 goes from {1, 2, 5, 8} to {2, 8} and column 5 from {2, 6, 8, 9} to {2, 8}. Now 1, 5, 6, and 9 have fewer places to go in the row, which often reveals a hidden single.

Why it is "hidden"

The pair is buried among other candidates, so the cells do not look special at first. Counting digits instead of looking at cells is the key to spotting it.