Sudoku technique · First needed at: Hard ●●●●○○○○

Hidden pair

Two numbers fit in only these two cells, so nothing else can go there.

An example from a real puzzle

How to spot it

1. In one group, look for two numbers that can each go in only the same two cells.
2. Those two cells keep just these two numbers: clear their other candidates.

Why it works

Both numbers must appear in the group and there are only two cells for them, so those cells are spoken for.