Sudoku
Every puzzle here has exactly one answer and can be solved by reasoning alone — no guessing, ever. Difficulty is measured by the hardest technique a solve actually needs, which is a far better guide than the number of clues.
Choose a difficulty
Why clue count is a bad measure of difficulty
Most sudoku sites rate puzzles by how many digits they start with. It is an easy number to produce and it is close to meaningless: a 26-clue puzzle can fall to nothing but obvious singles, while a 30-clue one can need an X-wing to crack. The clues tell you how much is missing, not how hard it is to work out.
So each puzzle here is handed to a solver that only knows human techniques — naked and hidden singles, naked pairs and triples, pointing pairs, box/line reduction, and X-wings — and is rated by the hardest one it had to reach for. Anything the solver cannot finish without guessing is thrown away rather than published.
The techniques, roughly in order
A naked single is a cell with only one candidate left. Ahidden single is a digit that fits in only one cell of a row, column or box. Those two alone will finish any easy puzzle.
A naked pair is two cells in a unit sharing the same two candidates — those digits cannot go anywhere else in that unit. Pointing pairsand box/line reduction are the two directions of the same idea: when a digit's possible places in a box all sit in one line, or its places in a line all sit in one box, you can eliminate it from the rest.
An X-wing is the first properly awkward one: a digit confined to the same two columns across two rows can be struck from those columns everywhere else. Expect it only at expert.