Learn / Sudoku
Sudoku rules, explained
Fill every row, column, and 3×3 box with the digits 1 to 9. Each digit appears exactly once in each unit. Start with the given numbers; never change them. A valid Sudoku puzzle has one solution. A number that fits a cell is only a candidate until the remaining constraints confirm it.
A checked example
Row 1, column 7 takes 3. Every other digit is already excluded by its row, column, or box.