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Hidden Pairs
 
     
 
Hidden Pairs

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.

Naked Pairs
Fig. 7.1
 

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.

Once we’ve cleared the board we can see some attacks that the new Naked Pairs have created:
Naked Pairs
Fig. 7.2
Hidden Pair Quiz

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

 

Naked Pairs Example
Fig. 7.3
 
 
 
 
 
 
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