Keep pencil marks for the unit you are studying.
Naked Pairs
Two cells, two candidates — clear those digits from the rest of the unit.
A naked pair is two cells in the same row, column or box that each contain exactly the same two candidates and nothing else. Those two digits must occupy those two cells in some order, so they can be removed from every other cell in that unit.
Worked example
27
27
1
9
8
How to apply it
Find two cells that show the exact same pair of candidates and no others.
Those two digits are confined to those two cells.
Erase both digits from the candidates of every other cell in that row, column or box.
The eliminations often reveal a naked or hidden single elsewhere.
When to use it
Naked pairs are the gateway to intermediate solving. Look for them on hard grids whenever singles stall — and remember the same idea scales up to naked triples and quads.