Hard Sudoku — play online free
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Settings
Keyboard: arrows — select a cell, 1–9 — digit, N — notes, Delete — erase, Ctrl+Z — undo.
Hard sudoku
Hard sudoku can't be solved by scanning for singles alone: at some point the easy moves run out. That's when pencil marks and elimination techniques help — pointing and claiming, naked and hidden pairs, triples. They don't place a digit directly but remove candidates, after which new singles appear.
Every hard puzzle on this site has been checked by a program: it has exactly one solution that can be reached by logic, without guessing. See How to solve sudoku for how these techniques work.