Naked Pair Sudoku Technique
The first technique that eliminates candidates instead of placing digits — and the gateway to everything above it.
Also called: naked double, conjugate cells.
What a naked pair is
Two cells in the same row, column or box hold exactly the same two candidates and nothing else. Between them they must use both of those digits, in one order or the other.
You do not know which cell takes which digit, and you do not need to. What matters is that both digits are consumed inside those two cells, so neither can appear anywhere else in the unit.
This is the first genuinely eliminative technique. It places nothing by itself; it clears candidates so that a naked or hidden single appears elsewhere.
How to spot one
- Scan each unit for cells showing exactly two pencil marks, then check whether any two of them show the same pair.
- The pair must be exact. Cells showing {3,7} and {3,7,9} are not a naked pair — the second cell has an escape route.
- A naked pair inside a box that also sits on one row is worth a second look: it often creates a pointing pair at the same time.
Worked example
| Column 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |
|---|---|---|---|---|---|---|---|---|---|
| Row 7 | 37 | 37 | 2 | 4 | 1379 | 6 | 157 | 8 | 579 |
Solid digits are already placed. Small digits are the candidates still available in that cell.
- The first two cells both show exactly {3, 7}. Between them they will use up the 3 and the 7 for the whole row.
- Strike 3 and 7 from every other cell in row 7. The fifth cell drops to {1, 9}, the seventh to {1, 5}, the ninth to {5, 9}.
- Nothing was placed, but three cells got smaller. That is what makes the next single findable.
What it eliminates
Both digits of the pair, from every cell in the shared unit other than the two forming it.
When you need it
Once grids drop to around 40 clues, singles alone start to stall and pairs are usually the first thing that breaks the deadlock.
Difficulty here is set by how many clues are left in the grid: Easy starts from 46, Medium 40 and Hard 34, with Extreme and Insane stripped further still. Fewer clues is what makes the techniques below necessary — the fewer digits you start with, the deeper you have to reason.