Naked pair
Two cells in one unit that can only hold the same two digits take both of them: no other cell in the unit can.
The idea
Suppose two cells in the same row, column or box both have exactly the candidates 3 and 7. One of them will be 3 and the other 7; we don't know which yet.
Either way, both digits are used up in that unit. So 3 and 7 can be crossed out of every other cell of the unit. When the two cells share a box and a line, the digits go from both.
How to spot it
- Look for two cells with the same two notes, in the same row, column or box.
- Also check cells with two notes against each other across a unit: they are easy to miss when far apart.
Worked example
Naked pair on a real puzzle
From our hard puzzle #22 (25 clues), after 25 earlier steps of our solver. Gold cells make the pattern; red notes are the ones it rules out.
How often our puzzles need it
Our solver works like a careful player: it always uses the easiest technique that still makes progress. Counting the puzzles where it had to use naked pair at least once:
- 276 of our 1000 hard puzzles
- 456 of our 1000 expert puzzles
- 379 of our 1000 master puzzles
The easiest level that can need it is hard Sudoku.