Hard Sudoku

Hard puzzles start with fewer numbers and can’t be solved with singles alone. Expect to use notes and to find eliminations before the next single appears. Par is 15:00.

How to solve a hard Sudoku

• Pointing. If every candidate for a number inside a box sits in one row, that number must go in that row within the box. Remove it from the rest of the row. The same works for columns.
• Box/line reduction. The reverse: if every candidate for a number in a row falls inside one box, remove that number from the other cells of the box.
• Naked pair. Two cells in the same row, column or box with the same two candidates, say 4 and 7. Those two numbers must go in those two cells, so remove 4 and 7 from every other cell in that unit. Triples work the same way with three cells.
• Hidden pair. Two numbers that can only go in the same two cells of a unit. Those cells can hold nothing else, so clear their other candidates.

Playing here

Tap a cell, then a number. Turn on Notes to pencil in candidates; they clear themselves when you place a number nearby. You get three hints per game and unlimited undo. Your game saves after every move, so you can close the tab and come back to it.

When these feel routine, Expert brings in X-Wing and XY-Wing.