How to find hidden pairs in sudoku

The hidden pairs, triples, and quads strategy involves looking for a group of candidates that can only occur in the appropriate number of squares, then eliminating other candidates from those same squares.

They are called "hidden" because there are other candidates in the same squares as the pairs, triples, and quads.

Hidden Pair

A hidden pair occurs when a pair of numbers appears in exactly two squares in a row, column, or block, but those two numbers aren't the only ones in their squares.

In the above example, the 5 and 9 in red are a hidden pair.

The 5 and 9 only occur in those two squares and nowhere else in the row, so those two squares can only contain 5 and 9 and no other numbers. Because of that, you can get rid of the other candidates in those squares, as shown crossed out in red.

Interactive Example

There is hidden pair in the following interactive example.