Naked Pair
The naked pair is one of the key clean-up techniques for hard Sudoku. Two cells in a unit hold exactly the same two candidates, and that is enough to thin out the whole unit. Try it interactively below.
Interactive example
Click ‘Step’ to see the pattern and the elimination.
Try it: Naked Pair
How the naked pair works
Find two cells in a row, column or block that hold exactly the same two candidates, for example 3 and 6. Which cell ends up with the 3 and which with the 6 is still open, but both digits are reserved for these two cells. So 3 and 6 cannot sit in any other cell of the same unit and can be removed there. This often opens the next naked or hidden single at once.
More techniques
Hidden Pair · X-Wing · Y-Wing