Hidden Pair
A Hidden Pair is the mirror image of a Naked Pair. Instead of two cells owning two digits, two digits own two cells: within one unit, both digits can only go in the same two cells. Since each of those digits must be placed somewhere in the unit, the two cells are reserved for them — and every other candidate in those two cells can be erased.
Here is the shape in practice. In the bottom-left box, suppose the digit 4 fits only in r8c1 and r9c2, and the digit 9 also fits only in r8c1 and r9c2 — everywhere else is blocked by placed 4s and 9s. Those two cells might currently show marks like 1/3/4/9 and 3/4/7/9, but they cannot afford to hold 1, 3 or 7: if either did, there would be no room left for both 4 and 9. So the marks collapse to 4/9 in each cell, and a hidden pair becomes a naked one.
The key distinction from a naked pair: a naked pair eliminates from the rest of the unit; a hidden pair eliminates from inside its own two cells. Hidden pairs are harder to see precisely because the extra candidates camouflage them — you find them by counting a digit's spots per unit, not by reading cell marks.
Hunt for them when a unit has two digits that each appear in only two places. They are especially valuable early, before full pencil marks, because cleaning two cells down to a bare pair often triggers naked-pair eliminations immediately afterwards.
- Count spots per digit per unit: any digit with exactly two homes is half of a potential hidden pair.
- Two such digits sharing the same two cells completes the pattern — then strip everything else from those cells.
- After the strip, treat the result as a naked pair and check the unit for follow-up eliminations.