1. Note candidates in the tight spots
Pick a row, column or box with only three or four open cells and write down (or keep in mind) which digits each cell can still take.
Naked pairs in sudoku
Stuck on harder sudokus with scanning alone? Then the naked pair is the next technique to learn. It is surprisingly simple, needs no arithmetic, and often unlocks a grid that seemed completely stuck.
What is a naked pair?
A naked pair is two cells in the same row, column or box that both have exactly the same two candidates — for example, both can only be 2 or 5. You don't yet know which cell gets the 2 and which the 5, but you know something at least as valuable: those two digits are certainly in those two cells. So you can remove 2 and 5 as candidates from every other cell in that same row, column or box.
Example: a row with a naked pair
Look at this row. The small digits are candidates: digits still possible in that cell.
4
2 5
7
2 5
1
2 5 9
3
6
8
The two green cells can each only be 2 or 5: a naked pair. Together they claim this row's 2 and 5. The third open cell had candidates 2, 5 and 9 — cross out 2 and 5, and only one possibility remains: 9. A naked pair hands you a digit outright, without guessing anything.
How to apply it, in three steps
2. Find two cells with exactly the same two candidates
If two cells both have precisely the same two candidates — not one more, not one less — you've found a naked pair.
3. Cross those two digits out elsewhere
Remove both digits as candidates from every other cell in the same row, column or box. Often a cell is left with a single candidate — and that one you can fill in.
Good to know
The same idea works with three cells and three digits: a naked triple. And mind the difference with the hidden pair: there, two digits can only go in two particular cells, but those cells still show extra candidates around them. The naked pair is bare — hence the name — which makes it the easiest to spot.
Try it yourself?
This article belongs to the overview of all sudoku techniques — it also shows which technique to learn next. Or apply the naked pair right away:
Frequently asked questions about naked pairs
What's the difference with a hidden pair?
In a naked pair, two cells have only the same two candidates. In a hidden pair, two digits can only go in two particular cells, but other candidates still sit alongside them. The consequence is reversed: with a hidden pair you cross out the other candidates inside those two cells.
Does the pair have to be in the same box?
No — a row, a column or a box is enough. If the pair happens to share both a row and a box, you may eliminate in both. That's extra profit.
Does this also work with three or four cells?
Yes: three cells sharing exactly three candidates form a naked triple, four cells with four candidates a naked quad. The principle is identical; the searching just gets a little bigger.
From which level do I need naked pairs?
Easy sudokus can be solved by scanning alone. From medium and certainly hard puzzles, situations appear where a naked pair makes the difference — it is the first 'advanced' technique every sudoku player learns.