Hidden Pairs are real work horses for removing candidates and they are the basis for more advanced strategies as well.  Simply put, if

Any two numbers occur only twice in just two cells in any one unit then all the other candidates can be removed from those cells. 

Let’s look at an example.  There are two separate Hidden Pairs in the top half of this Sudoku, marked in the two different rings.

1 and 5 occur in only two cells in Box 2 but they are hidden by the 3 and 8 in C4 and the 1/3/7 in C6.  Those numbers can go.  Similarly in the centre box we have two occurrences of the pair 3/9.  D5 and F5 can loose 1/5.

Once you’ve exposed a Hidden Pair it becomes a Naked Pair and you can go back to the Naked Pair strategies and see what damage can be done elsewhere on the board.

How many Hidden Pairs are in this Sudoku? Answer on page