How to solve a hidden pair in Sudoku
A hidden pair is two digits that can only go in the same two cells of a unit, so those cells can hold nothing else.
How it works
If, in one unit, the digits 4 and 8 each have only two places left, and they are the same two cells, those cells must hold the 4 and the 8.
Every other candidate can then be removed from those two cells. The pair is hidden because the cells may list many other candidates that hide it.
How to spot it
In a unit, find digits that appear in just two cells. If two digits share the same two cells, you have a hidden pair.
A worked example
This is a step from a real puzzle, a moment where the next move needs a hidden pair.
Struck-through digits are the ones that can be removed.
In row 7, 2, 4 can only go in R7C2, R7C4, so those cells can hold nothing else. Remove the other notes from them.
Where it fits in Rocksled Sudoku
Puzzles that need a hidden pair are never rated below Hard. Each step of this kind adds 24 points to a puzzle’s difficulty score. When you ask for a hint that needs it, the game names the technique and shows the cells involved. See how to play for the rest of the controls.