Naked pairs in Sudoku: two cells that eliminate two digits
See exactly when two matching candidate sets form a naked pair, where you can erase their digits, and when the pattern does not apply.
The rule for a naked pair
A naked pair consists of two empty cells in the same row, column or box, each with exactly the same two candidates. Those two cells must use the two digits between them. You may therefore remove both digits from every other cell in that shared region.
You do not know which cell contains which digit. A pair is useful because it eliminates options elsewhere, not because it immediately solves its two cells.
A worked row example
Suppose the complete notes at r4c1 and r4c4 are both {2, 6}. Here r4c1 means row 4, column 1. In either possible order, those cells occupy 2 and 6 in row 4.
If r4c6 has {2, 6, 9}, remove 2 and 6 there, leaving 9. The table shows only the relevant cells of a hypothetical row; other cells are omitted.
| Cell | Before | After |
|---|---|---|
| r4c1 | 2, 6 | 2, 6 |
| r4c4 | 2, 6 | 2, 6 |
| r4c6 | 2, 6, 9 | 9 |
Keep the elimination inside the shared region
The pair in the example shares row 4, so its eliminations apply to row 4. It does not let you remove 2 and 6 from every cell in columns 1 and 4. Either digit could still appear elsewhere in those columns.
If both pair cells also lie inside the same box, the same argument applies to other cells in that box. Identify the common region before erasing anything.
Three similar-looking patterns that are not this pair
A cell with {2, 6} beside a cell with {2, 6, 9} is not a naked pair: the second cell can still be 9. Two matching sets in unrelated cells do not form a pair either.
If three cells in one region all have only {2, 6}, the current notes or entries are inconsistent: two digits cannot fill three cells. Do not choose two arbitrarily. A hidden pair is different again; it is defined by the only positions for two digits, even when those cells contain extra notes.
Try the deduction
Assume the notes are complete and all three cells are in the same column.
A has {3, 8}, B has {3, 8}, and C has {1, 3, 8}. What can you write in C, and can you yet distinguish A from B?
Show the explanation
C must be 1 because A and B occupy 3 and 8 in that column. A and B remain unresolved: either order is still possible.
A useful scanning routine
After checking singles, scan one region for cells with exactly two candidates. Compare those short lists. When a pair produces an elimination, check singles again before looking for a harder pattern.
On iPlay Sudoku, use Notes to track the two candidates and manually remove proven exclusions. Medium is a useful starting setting, although not every generated puzzle requires a pair.