Hidden Pair
Intermediate Removes candidates Used in: Sudoku, Killer Sudoku, Calcudoku, Futoshiki
Two digits appear as candidates in only two cells of a unit. Those two cells must take those two digits, so every other candidate can be cleared out of them. The pair is “hidden” because each of the two cells also shows extra pencil marks that turn out to be impossible.
How to spot it
Find two digits whose only possible homes in a unit are the same two cells.
What you can conclude
Strip every other candidate out of those two cells.
Worked example
If 3 and 7 can only appear in two cells of a row, and one of them also shows {3,4,7,8}, drop the 4 and 8 — it is 3 or 7.