Step 8 · Hard
What is Naked Pair? How to Solve Killer Sudoku
When two cells in the same row, column, or box have the same two candidates and nothing else, you can remove those two digits from every other cell in that group.
What it is
If two cells in the same row (column or box) can each only be 3 or 8, then 3 and 8 are used up by those two cells — even if you don't know which is which yet.
So no other cell in that row can be 3 or 8.
How to spot it
When you find a cell with only two candidates, look in its row, column, and box for another cell with exactly the same two.
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 7, R8C1 and R9C1 can only hold 2 and 8. That's 2 cells for 2 digits, so 2 and 8 are used up here (even if we don't know which goes where yet). So the other cells of box 7 (R7C2, R7C3, R8C2, R8C3, and R9C3) can't be 2 and 8.