Two digits fit in only the same two cells → erase every other candidate from those cells.
Technique 5 in the order the hint searches · difficulty Hard · up to pairs
This is the naked pair turned inside out. If, in some unit, the cells that could take a 2 and the cells that could take a 7 are the same two, then those two cells belong to the 2 and the 7, however many other candidates happen to be written in them.
Count each digit's homes and look for a matching pair
For one unit, count for each of 1 to 9 how many cells it could go in. When several come out at two, look for a pair whose two cells match exactly. This counting is dull enough that people rarely do it by eye.
How the hint says it
In the top-right box, 3 and 4 can only go in r3c7 and r3c9. Those two digits share those two cells, so the leftover candidates 2, 5, 8 and 9 come off them.
Two digits have reserved two cells
Both the 4 and the 9 must appear somewhere in that unit, and there are only those two cells for them. The two cells have to be filled by 4 and 9, so any other candidate written there has nowhere to stand. Nothing gets placed, but both cells thin out sharply.
Hard, the same as the naked pair. The difference is that this one happens inside cells crowded with candidates, so the eye finds it far later.
Puzzles that need this technique
Four puzzles with a spot where nothing moves until you use Hidden Pair. Press a number to play it.
- Practice 1 ·
2-9435761 - Practice 2 ·
2-7743044 - Practice 3 ·
2-7178805 - Practice 4 ·
2-3793371