Fill every row, column and 3×3 box with the digits 1 to 9, once each. Click a square and type a digit, or use the pad below. Press N or the notes button for pencil marks, and backspace to clear a square. Duplicates light up red as you go. Every puzzle is dug out of a seeded grid and checked to have exactly one answer, so a solve never needs a guess, and the seed plus your keystrokes are replayed server-side before a time reaches the global leaderboard.