Hidden Pair
Two digits can only go in the same two cells of a unit, so every other candidate is cleared from those cells.
Rule
In a unit, x and y appear only in cells A and B → A and B keep only {x, y}.
Find two digits with only two places
In box 4, digit 3 can only go in r4c3 and r6c3; digit 8 can only go in those same two cells.
The idea
A hidden pair is seen from the digits' side rather than the cells'. If in a column the 2 can only go in cells A and B, and the 6 can only go in those same two cells, then A and B must be 2 and 6 in some order.
Both cells are taken by 2 and 6, so every other candidate in A and B goes. It is called hidden because the extra candidates hide it; once they are cleared, the hidden pair becomes a naked pair.
How to spot it
- For each unit, count how many places each digit has left. Two digits that each have exactly two places, and the same two, form a hidden pair.
- They show up in units with many empty cells, where naked pairs are rare.
Common mistakes
- Both digits must have exactly two places, and the same ones; a digit with three places does not qualify.
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Related techniques
References
The examples on this page are generated from random puzzles, not taken from these sources.