Fill every square so that each row, each column and each of the nine three by three boxes holds the digits 1 to 9 exactly once. Every puzzle here has exactly one answer, so the answer is always something you can reason out.
Notes let you write small candidate digits in a square while you work. A digit you place clears itself from the notes of every square it can see. A digit that repeats in its row, column or box turns red. A hint fills one square, and a best time needs a solve with none. Your game is saved on this device, so closing the tab does not lose it.
Click a square, then type a digit. Arrow keys move, Backspace erases, P turns notes on and off, U or Ctrl+Z undoes, I is a hint, N is a new puzzle, M mutes, F is fullscreen.
Tap a square, then tap a digit underneath. Notes turns the digits into small pencil marks. The number under each digit is how many of it are still to place.