Naked pair
Two cells holding the same two candidates lock them in the unit
How to use it
Two cells in one unit whose pencil marks are the same two symbols. Between them they must take exactly those two symbols — so every other cell of the unit loses both.
Spot them during candidate scans: matching two-value marks sharing a row or column. On the letter layer the symbols are letters, on the digit layer digits. Naked pairs pay off twice: the two eliminations themselves, and the candidate reductions that may naked-single another cell of the unit.
Why it is sound
Two cells of a unit holding only symbols x and y must take x and y in some order (both symbols must appear in the unit, and no other cell may supply them once the two cells are accounted for). Any other cell of the unit taking x or y would leave the pair unfillable, so both eliminations hold in every solution.
A worked example
In column 1, the letters of r1c1 and r2c1 have both been reduced to exactly {A, C}. Between them the two cells must take A and C — one each — so no third cell of the column can host either letter: A and C are struck from r3c1. Note the shape: the pair cells may sit anywhere in the unit — they need not be adjacent — and the marks must match exactly. After the strike, r3c1's letter candidates shrink and the column often hands you the next single.