Core techniques
The candidate-based moves that crack Medium puzzles.
Core techniques — what "Medium" means
Medium puzzles need techniques that eliminate candidates across several cells before any single is revealed. They all operate on your notes.
Naked pair
If two cells in the same unit share the same two candidates and nothing else (say, only 3 and 7), then 3 and 7 are locked to those two cells — you can erase 3 and 7 from every other cell in that unit.
Two cells in this row both show only {3, 7}. So 3 and 7 can be removed from the other cells in the row, often exposing a single elsewhere.
Hidden pair
The mirror image of a naked pair: if two digits can each only go in the same two cells of a unit, those cells form a pair — you can erase every other candidate from those two cells.
In this box, the digits 2 and 5 can each only land in the same two cells. Those two cells are therefore {2, 5}, and any other candidates in them are removed.
Pointing pair
Every box must contain each digit somewhere. So when a digit's only possible cells in a box all fall on one row (or column), that digit is pinned to that line — and since each row and column holds the digit just once, it can be erased from the rest of that row (or column) outside the box.
Every box must hold a 6 somewhere. Inside this box, the only cells that can hold a 6 lie in a single row — so 6 is removed from that row everywhere outside the box.