Hidden Pairs

A Hidden Pair occurs when two specific numbers appear as candidates only in two cells within the same row, column, or 3x3 block. These cells may also contain other candidates (which is why the pair is "hidden").

Even with other numbers present, we can deduce that those two candidates must necessarily occupy those two cells. Consequently, we can eliminate all other candidates from those two cells, leaving only the Hidden Pair.

In the example above, let's analyze the center left block. We notice that the candidates 2 and 6 appear only in two cells: R5C1 and R6C1. Even though these cells contain other numbers (like 3, 4, 7, 8), the 2 and 6 are unique to those positions. We can then remove all other candidates from R5C1 and R6C1, leaving them with the naked pair 2 and 6.