Step 9 · Hard
What is Hidden Pair? How to Solve Killer Sudoku
When two digits can only go in the same two cells of a row, column, or box, you can remove all other candidates from those two cells.
What it is
If, in a row (column or box), both 2 and 6 can only go in the same two cells, those two cells must be 2 and 6.
So you can remove every other candidate from those two cells.
Unlike a Naked Pair, the cells may show lots of candidates — but 2 and 6 have nowhere else to go.
How to spot it
In a group, look for a digit with only two possible spots, then check whether another digit has exactly the same two spots.
Example
Dotted outlines are cages, with their total in the top-left corner. Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the green digit is the answer.
In box 4, both 7 and 8 can only go in R4C2 and R5C2. Those two cells must be 7 and 8, so remove their other candidates (2 and 6).