A naked pair is two cells in the same house that hold exactly the same two candidates — and nothing else. Between them the two cells must take both digits (one each, in some order), so every other cell in that house can drop those two digits from its pencil marks.
To spot one, look for twin cells: two cells in a row, column, or box showing the same tight pair of pencil marks. The pair does not tell you which cell gets which digit — it does not have to. The claim is on the digits, not the order.
It works by counting. Two cells, two candidates between them: if either digit went anywhere else in the house, one of the pair cells would be left with nothing legal to hold.
On the board
In column 7 of this board, the cells at rows 5 and 6 both show exactly 1 and 4. Those two cells must take the 1 and the 4 between them, so the rest of the column surrenders both digits: R1C7 drops 1 and 4 (keeping 7 and 8), R7C7 drops its 4, and R9C7 — which held 1, 3, and 4 — drops both, collapsing to a lone 3. The pair never told us its order, and it did not need to: it still handed us a free naked single.
This is the exact board the app's interactive Naked Pair demo uses — step 12 of its 67-step walkthrough, drawn here with its live pencil marks.
