Rules & objective
Fill every row, column and 3×3 box with 1–9 without repeats. Every puzzle has one solution. Levels have 30, 40, 50 or 60 blanks; clue count is not a technique rating. Select a cell by tap or arrows, then enter a digit; original givens are immutable. Freely replace or clear mistakes, with no lives. Hints fill one correct digit and record assistance; revealing the full answer ends this run with an assisted result. Completion counts correctly filled blanks. Rules and hidden pages pause the timer. New draws a fresh puzzle; zoom permits board scrolling.
Hints, answer checks and automatic candidates count as assistance; Undo and Restart retain that mark.
Controls
Arrows choose cell · 1–9 enter · 0/Delete clear
Worked example
If the row, column and box together exclude 1–8, that cell must be 9. Checking the row alone is not enough to establish the answer.