Hidden Pair
Two digits whose only possible homes within a unit are the same two cells. Eliminate every other candidate from those cells.
Intermediate sudoku technique
What it is
A hidden pair is the mirror of a naked pair. Instead of finding two cells with the same 2-candidate set, you find two DIGITS whose only viable cells in a unit are the same two cells. Those two cells must hold those two digits between them — so any other candidates currently sitting in those cells get eliminated. Hidden pairs are easier to overlook than naked pairs because the two target cells often have busy candidate lists. Scanning by digit, not by cell, is the key skill.
When to use it
After naked pairs are exhausted. For each unit, walk through every digit pair and ask: do these two digits share the same two cells? If yes, prune.
Worked example
In box 2 (top-middle), the digit 4 can only appear in R1C5 and R2C6. The digit 9 can also only appear in R1C5 and R2C6. So {4, 9} is a hidden pair in those two cells. R1C5 currently has candidates {3, 4, 6, 9} and R2C6 has {4, 7, 9} — we eliminate {3, 6} from R1C5 and {7} from R2C6, leaving each cell with just {4, 9}.
Try it
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