The pairs technique says: when two of the same number sit side by side, one of them stays unshaded (they can’t both be shaded, since they’d touch). So any third copy of that number in the same line is shaded.
This is the fourth of the Hitori techniques.
The rule
- Find two of the same number side by side in a row or column.
- Look for a third copy elsewhere in that same row or column.
- Shade it.
In a real puzzle
On the 9×9 every technique page follows, it’s first needed at step 24. Column 3 has two 2s side by side, in rows 6 and 7, and a third 2 at the bottom, in row 9. That one is shaded.
How often you’ll need it
Rarely as the move to make: on 9 of the 50 Easy levels, 12 Medium and 14 Hard, usually once. Often a sandwich or the chain gets there first. But when nothing else is moving, a pair with a third copy is quick to spot. On the 9×9 here it’s 2 of the 55 moves.