The rules, made simple
What counts as a valid move—and what does not.
Use every digit once.
A completed row must contain 1, 2, 3, 4, 5, 6, 7, 8, and 9. The same is true of every column and 3 × 3 box. You do not add the digits together to find an answer; you eliminate possibilities using the three constraints.
Fixed clues stay fixed.
The darker, heavier numbers are part of the puzzle. Your entries are shown in the accent color. Erasing or replacing a given clue would change the puzzle, so the game keeps those cells read-only.
A legal-looking move can still be wrong.
A digit may not repeat immediately in its row, column, or box and still lead to a contradiction later. Local conflict highlighting only catches duplicates. A logically justified placement is stronger than a number that merely fits right now.
One puzzle, one solution.
Our published puzzle packs are checked for exactly one solution. When every cell is filled and all three constraints are satisfied, you have finished. Notes are a working aid, not part of the final answer.
Diagonal lines are not special.
This is classic Sudoku. The two long diagonals do not need to contain unique digits. Additional diagonal constraints belong to a separate Sudoku variant and do not apply here.