Naked Pairs and Naked Triples
When two cells in the same unit can only hold the same two numbers, those numbers are spoken for. Nothing else in that unit can use them.
What is a naked pair?
A naked pair is two cells in the same row, column or box that each have exactly the same two candidates, for example both {2, 6}. One of the cells will be 2 and the other will be 6. We don't know which yet, but we do know that 2 and 6 are both used up inside this unit.
That lets you remove 2 and 6 from every other cell in the same unit.
The pair / patternCandidates removedUnit being checked
In this row, the second and seventh cells both have only 2 and 6 as candidates. The other two empty cells had 2, 5, 6 and 1, 6 as candidates. After removing the pair's numbers they are left with a single candidate each, 5 and 1, which are new naked singles.
Naked triples
The same idea works with three cells and three numbers. If three cells in one unit contain only candidates from the same set of three numbers, those three numbers can be removed from the rest of the unit.
The tricky part is that each cell doesn't need all three numbers. For example, cells with {1, 4}, {4, 8} and {1, 8} form a naked triple on 1, 4 and 8, even though no single cell contains all three. What matters is that together the three cells use only three different numbers.
Naked quads (four cells, four numbers) exist too, but they are rare and usually easier to find as a hidden pair in the remaining cells.
How to spot them
- Your notes need to be complete and up to date. A naked pair is invisible without candidates.
- Look for cells with only two candidates (bivalue cells), then check whether another cell in the same row, column or box has exactly the same two.
- Remember that a pair can share a box and a row. Then you can clear candidates from both units at once.
Don't confuse it with a coincidence: two cells with {2, 6} in different rows, columns and boxes don't form a pair. The two cells must share a unit.