Naked Pairs

A Naked Pair occurs when two cells within the same row, column, or 3x3 block contain the exact same pair of candidates (and no others). For example, both cells only contain the candidates 7 and 9.

The logic is that if 7 and 9 must occupy those two cells, then they cannot appear in any other cell of the same unit (row, column, or block). We can therefore eliminate the candidates 7 and 9 from all other cells in that unit.

In the example above, the highlighted yellow cells (R5C3 and R5C8) form a Naked Pair. Both contain only 7 and 9. Since they are in the same row 5, we can safely remove 7 and 9 as candidates from all other cells in that row.