Naked Pairs
A Naked Pair is two cells in one row, column, or box whose combined candidate set contains exactly two digits. The two cells reserve those digits, so the digits can be removed from other cells in that sector.
What is a naked pair?
Count distinct candidates in the union of the two selected cells. A common irreducible example is {A,B} and {A,B}; in a complete sound candidate state, a distribution such as {A} and {A,B} is already resolved by the single and should be handled at that simpler step.
Eliminate candidates
Within one sector, find two unsolved cells whose combined candidates are limited to the same two-digit union. Then look outside the selected cells, but within that sector, for occurrences of either digit.
Look for related constructions
Two cells need two different values and can use only the two digits in their union. They therefore occupy both digits between them, so neither digit can occur in another cell of the same sector.