We can easily extend Hidden Pairs to Hidden Triples or even Hidden Quads. A Triple will consist of three pairs of numbers lying in three cells in the same unit (row, column or box), Such as 4/8/9, 4/8/9 and 4/8/9 hidden behind other candidates However, Hidden Triples are more interesting and follow a similar idea as Naked Triples. We don't need exactly three pairs of numbers in three cells for the rules to apply. In the example below we have 1/4/6, 1/4 and 1/4/6 in three cells.
Fig. 8.1
Hidden Quads
Just like Naked pairs we can extend Hidden Pairs and Triples to Quads and even beyond.
These are extremely hard to spot!
Fig. 9.1
In column 8 we have a Hidden Quad with the numbers 3/5/6/7 spread over B8, D8, E8 and F8. The Quad is barely hidden – just by one number. But logic ensures that the 4 in B8 can be safely removed, and ensure that Sudoku can be solved.
