Step 6 · Hard
What is Hidden Pair? How to Solve 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
Yellow cells are where to look, orange cells are the reason, red digits are candidates you can remove, and the blue digit is the answer.
In box 8, both 4 and 8 can only go in R7C5 and R8C5. Those two cells must be 4 and 8, so remove their other candidates (7 and 9).