Hard Sudoku, Play Free Online, 52 Empty Cells

Hard sudoku is where simple elimination stops being enough: 52 of 81 cells are empty, and you'll need the "pointing pair" technique. Picture a 3×3 block where the digit 5 can only sit in two cells, and both happen to be in the same row of that block, because every other cell in the block is already filled. That means 5 must appear somewhere in that row within the block, so you can eliminate it from every other cell in that same row outside the block, even in neighboring blocks. That conclusion comes from looking at a whole line, not a single cell, and that's what separates hard from medium. A grid takes 20-30 minutes, and some cells won't get filled right away, you'll first pencil in candidates and narrow them down. Breezing through it? Move on to expert sudoku. Need to shore up the basics first? Go back to medium sudoku.