Hidden Pairs
A hidden pair is the mirror image of a naked pair. Instead of two cells with the same two candidates, you find two numbers that can only go in the same two cells.
The idea
Look at one unit (a row, column or box) and pick two numbers. If both numbers have exactly the same two possible cells in that unit, then those two cells must hold those two numbers. There is no room left in them for anything else.
The cells may still have extra candidates written in them, which is why the pair is "hidden". Once you spot it, you can erase every other candidate from those two cells, turning the hidden pair into a naked pair.
Unit being checkedThe pair / patternCandidates removed
In the middle box, 3 appears as a candidate only in the top-left and bottom-right cells, and so does 8. The box needs a 3 and an 8, and these are the only two cells that can take them, so those cells are {3, 8} in some order. The extra candidates 1, 5 and 6 are crossed out.
Why this helps
Removing those extra candidates often has a knock-on effect. In the example, 1, 5 and 6 now have fewer places to go in the box and in the cells' rows and columns, which can create new hidden singles or pointing pairs.
How to spot hidden pairs
- Instead of reading cells, read numbers. For one unit, count how many cells each missing number could go in.
- Any number with exactly two possible cells is worth noting. If another number has the same two cells, you have a hidden pair.
- Hidden pairs show up most often in units with many empty cells, where naked pairs are hard to see.
Hidden triples
With three numbers that are confined to the same three cells of a unit, you get a hidden triple, and you can remove all other candidates from those three cells. As with naked triples, each number doesn't need to appear in all three cells, as long as together the three numbers are limited to those three cells.