Hard is where sudoku starts feeling like a logic puzzle instead of a search game. The starting grid leaves enough whitespace that you'll hit a wall partway through — usually with 20-30 cells left — and have to pull out a more advanced pattern to break it. Once you crack it, the rest collapses fast.
Techniques you’ll use
→Everything from Medium, plus:
→Naked pairs and triples: two or three cells in a unit whose candidates are exactly two or three matching digits, eliminating those digits from other cells in the unit
→Box-line reduction: a candidate restricted to one row or column within a box also gets eliminated from that row or column outside the box
→Locked candidates (the inverse of pointing pairs)
When to play it
Best for a quiet 20 minutes when you want to actually think, not just solve.
Difficulty context: Two to three times as hard as Medium. Most players settle here for their daily.