Naked Pairs in Sudoku

What it is

A naked pair is two cells in the same unit — row, column or box — that contain exactly the same two candidates, and nothing else. If two cells can only be 4 or 7, then one of them will be the 4 and the other the 7. You don't know which yet, and you don't need to: those two values are spoken for. No other cell in the unit can take them, so every 4 and every 7 pencilled elsewhere in that unit can be erased.

It's the first technique that doesn't place a digit but removes candidates — and it's often precisely one of those removals that unlocks the next placement.

How to spot it, step by step

1. Pick a unit with several empty cells and up-to-date pencil marks.
2. Look for cells with exactly two candidates. Only these can form the pair.
3. Compare those cells with each other: you need two with the same two candidates, identical.
4. Once you've found the pair, sweep the rest of the unit and erase both values from every pencil mark.
5. Check the fallout: if a removal leaves some cell with a single candidate, you've earned a naked single, ready to place.

The example in the diagram

In the diagram's row, the second and fifth cells both contain only the candidates 4 and 7: that's the naked pair. The third cell had 2, 4 and 9 pencilled in — the 4 goes, leaving 2 and 9. The sixth had 3, 7 and 8 — the 7 goes. The pair itself stays unresolved, and that's fine: its job is to clean the row.

Common mistakes

The most frequent: mistaking for a pair two cells that share two candidates but hold others too. A cell with 4-7 and a cell with 4-7-9 do not form a naked pair — that's, if anything, the road toward a hidden pair. The second: erasing candidates outside the unit. The pair constrains only the unit that contains it; if the two cells happen to share both a row and a box, then — and only then — the elimination applies to both.

Where you'll need it

The naked pair marks the entrance to medium sudoku: it's the technique that opens the tier, and it teaches the mental move behind everything that follows — not "which digit goes here", but "which digit can no longer go there". The same principle later stretches to three cells and three candidates, the naked triple, which you'll meet from the hard level on. It works hand in hand with Pointing Pairs, the other cornerstone of the tier; the full picture is in the technique guides index.